Chapter 7: PLACENTA, CORD, AND MEMBRANES

The placenta, cord, and membranes are the building blocks of human pregnancy. Vast changes take place in the first trimester and throughout pregnancy that can be followed sonographically. The placenta provides the essential connection between the mother and developing fetus. Many clinical problems are attributed to the placenta, despite the fact that they cannot always be explained after pathologic examination.

A thorough understanding of the anatomy of the normal placenta and its variations, as well as the pathologic conditions that are known to occur, is necessary to correctly interpret the sonographic appearance. Placental location with respect to the internal cervical os and the maternal urinary bladder will be reviewed, especially in light of the increasing cesarean delivery rate and abnormal placentation. The umbilical cord and membranes share similar developmental origins, which will be considered. Intrauterine membranes can be delineated with sonography in patients who have bleeding episodes and in uncomplicated pregnancies. Their assessment is clinically important in multifetal pregnancies.

This chapter examines the embryology of the placenta, cord, and membranes. Normal and abnormal processes of placentation and umbilical cord development will be reviewed. Pertinent clinical aspects are highlighted.

Decidual Change

The endometrium undergoes changes in preparation for embryo implantation known as the decidual reaction or decidualization, where secretory endometrium is transformed into deciduas.^1,2 This change is prompted by estrogen, progesterone, and other factors secreted by the invading blastocyst.² There are 3 parts of the decidua: (1) decidua basilis, the modified portion of decidua directly beneath the implantation site; (2) decidua capsularis, the portion overlying the blastocyst; and (3) decidua parietalis, the portion covering the remainder of the endometrium (Figure 7-1). Initially, there is a gap between the decidua capsularis and decidua parietalis since the gestational sac does not fill the entire uterus.² The intradecidual sign describes the early sonographic appearance of the gestational sac that does not deform or displace the central uterine cavity (Figure 7-2A). As the gestational sac continues to grow into the central uterine cavity, the double decidual sac sign is seen (Figure 7-2B) with the echogenic decidua capsularis and peripheral decidua parietalis.³

Fertilization and Implantation

Fertilization of the ovum usually occurs in the ampulla of the fallopian tube, and the compact ball of cells or morula, containing an inner cell mass and an outer cell mass, reaches the uterine cavity 3 days after fertilization (Figure 7-3). The morula is converted to a blastocyst as fluid enters from the uterine cavity, and the blastocytic cavity is divided into 2 parts. The outer cell layer proliferates to form the trophoblast or embryonic portion of the placenta, and the inner cell mass forms the embryo.^2,4,5

The trophoblast proliferates once the blastocyst attaches to the endometrial tissues at about 6 days after fertilization and differentiates into an inner layer (cytotrophoblast) and outer layer (syncytiotrophoblast). A space in the blastocyst forms, which is the early amniotic cavity. Syncytiotrophoblastic cells secret erosive enzymes allowing the blastocyst to completely implant and erode endometrial blood vessels by the end of the second week (Figure 7-4). Lacunae in the syncytiotrophoblast coalesce to form lacunar networks at the embryonic pole, forming the intervillous space. Chorionic villi invade the decidua basilis and enlarge the intervillous space. As maternal vessels are further eroded by syncytiotrophoblast, maternal blood bathes the lacunar network.¹

Placenta and Fetal Membranes

Differentiation of the trophoblastic cells from the embryo precursors occurs at approximately 5 days after conception. The trophoblast is differentiated into 2 components: the cytotrophoblast and the syncytiotrophoblast. The former is mononuclear, and the latter is multinuclear. The syncytiotrophoblast secretes human chorionic gonadotropin and is responsible for proteolytic invasion into the decidua. As trophoblastic tissue proliferates, chorionic villi cover the entire chorionic sac. With continued sac growth, villi of the decidua capsularis are compressed and degenerate, yielding the chorion laeve or smooth chorion, which eventually becomes the chorionic membrane. The villous chorion (chorion frondosum) or the fetal portion of the placenta is anchored to the maternal component (decidua basilis) by the cytotrophoblastic shell (Figure 7-5).⁶ The chorion frondosum can be recognized sonographically in the first trimester as a thickened region of echogenic tissue bordering a portion of the gestational sac, indicating the site of the developing placenta. As the pregnancy grows, the decidua capsularis projects into the uterine cavity and fuses with the decidua parietalis, eliminating the uterine cavity. Sonographically, this is depicted as a gestational sac (Figure 7-6). Clinically, it is important to remember that this membrane fusion is a weak attachment, and blood from the intervillous space can dissect the fused chorionic membrane away from the decidua parietalis, yielding hematoma formation (Figure 7-7A,B, and C). The amniotic sac fuses with the chorionic leave, forming the amniochorionic membrane (Figures 7-8A and B, and 7-9).⁶

Spiral endometrial arteries in the decidua basalis bathe the intervillous space with maternal blood for nutrient and gas exchange. Endometrial veins drain this space (Figure 7-10).⁶ Deoxygenated fetal blood reaches the placenta through the 2 umbilical arteries, and oxygenated blood returns to the fetus through the single umbilical vein.

The early gestational sac is covered by chorionic villi, which are visible by transvaginal sonography as a hyperechoic rim at about 4 to 41/2 weeks' menstrual age (Figure 7-11A). The early placenta formed by proliferation of villi is seen in Figure 7-11B. By 9 to 10 weeks, the diffuse granular echotexture of the placenta is clearly apparent at sonography. This texture is produced by echoes emanating from the villous tree, which is bathed in maternal blood (Figure 7-11C). The placenta retains this general sonographic appearance throughout the pregnancy, with the notable exception of calcium deposition.

Placental Calcification

Placental calcium deposition is common, may be extensive, and is a normal physiologic process that occurs throughout pregnancy.⁷ During the first 6 months, the calcification is microscopic; macroscopic plaques typically appear in the third trimester, most commonly after 33 weeks.^8,9 The calcium is deposited primarily in the basal plate and septa, but is also found in the perivillous and subchorionic spaces.⁷ Plaques of calcium are readily detected at sonography as echogenic foci that do not produce significant acoustic shadows (Figure 7-12). The circular configuration seen in heavily calcified placentas results from septal calcifications.

Placental calcification has been studied by histologic,^10,11 chemical,¹² radiographic,⁸ and sonographic⁹ techniques with the following conclusions. The incidence of placental calcification increases exponentially with increasing gestational age, beginning at about 29 weeks (Figure 7-13).^{8,9, and 10,12} More than 50% of placentas show some degree of calcification after 33 weeks.⁹ There is no increased calcification in postmature placentas.^10,12 Placental calcification is more common in women of lower parity^{8,9, and 10} and is probably related to maternal serum calcium levels.^7,13 Placental calcification is also more common in late summer and early fall deliveries, when maternal serum calcium levels are highest.^12,13 There is no proof that placental calcification has any pathologic or clinical significance.^7,14,15

Classification of macroscopic placental lesions can be grouped by their appearance, their pathogenesis, or their functional significance.⁷ As such, placental lesions may be due to (1) disturbances of maternal blood flow to or through the placenta; (2) disturbances of fetal blood flow to or through the placenta; (3) thrombi and hematomas; and (4) nonvascular lesions.⁷

Maternal blood pooling in the perivillous and subchorionic space is a normal finding visualized sonographically as anechoic or hypoechoic areas within the placenta. Slow flow can be demonstrated in these spaces. Eventually, fibrin deposition occurs. Macroscopic lesions that have been diagnosed sonographically consist of subchorionic fibrin deposition, intervillous thrombosis, perivillous fibrin deposition, and septal cysts (Figure 7-14).

Subchorionic Fibrin Deposition

A layer of subchorionic fibrin is noted in the majority of placentas.⁷ These subchorionic anechoic or hypoechoic areas are visualized in approximately 15% of obstetric sonograms (Figure 7-15) and correspond to areas of subchorionic fibrin deposition in the term placenta.¹⁶ Histologically, the fibrin is attached to the underside of the chorionic plate and is usually not clinically significant.⁷ Subchorionic fibrin refers to a laminated collection of fibrin between the chorionic plate and placental villi. It is a result of pooling and stasis of maternal blood in the intervillous space beneath the chorion, which leads to thrombosis and secondary fibrin deposition. Slow flow is often visible with real-time sonography (Figure 7-16). Subchorionic blood pooling and fibrin deposition may be quite prominent at sonography (Figure 7-17); it is important to recognize this entity so as not to confuse it with a chorioangioma.

Intervillous Thrombosis

Intervillous thromboses are intraplacental (villous-free) areas of hemorrhage, entirely within the intervillous space, with a variable gross appearance that depends on the age of the lesion.⁷ Fresh lesions are dark red, but with aging change to brown, yellow, and finally white. Usually, there are visible laminations that microscopically consist of layers of fibrin. Both fetal and maternal red blood cells are present, suggesting that a leakage of fetal cells from a villous tear stimulates maternal coagulation.¹⁷ Intervillous thromboses have a reported incidence that varies widely in the literature (3% to 50%) in term, uncomplicated pregnancies.⁷

At sonography, intervillous thromboses appear as anechoic or hypoechoic intraplacental lesions that vary in size from a few millimeters to several centimeters (Figure 7-18).^18,19 No visible flow is identified sonographically. They may extend to the subchorionic space or the basal plate. These thrombi have no effect on placental function, but pinpoint a site of bleeding into the intervillous space.⁷

Some authors have suggested that intraplacental lesions may be associated with elevated maternal serum α-fetoprotein (MSAFP) levels in patients with normal-appearing fetuses^{20,21, and 22}; however, with the exception of a case report documenting an intervillous thrombosis in a patient with elevated MSAFP,²³ these studies do not provide pathologic documentation regarding the lesions seen at antenatal sonography. If the correlation is valid, it should be true only in the case of intervillous thromboses, because these are the only macroscopic placental lesions that are known to contain fetal blood.

Perivillous Fibrin Deposition

Perivillous fibrin deposition results from pooling and stasis of blood in the intervillous space. The lesions consist of nonlaminated plaques, varying in color from brown to white depending on age (Figure 7-19). Sonographically, they appear as intraplacental anechoic or hypoechoic lesions. Almost all full-term placentas contain some degree of perivillous fibrin deposition, visible macroscopically in most as fine speckling.⁷ In a percentage, the fibrin deposition is more extensive, forming a large, plaque-like lesion. Perivillous fibrin deposition and plaques are of little clinical significance.⁷

Maternal Lakes

"Maternal lakes" are anechoic lesions in the placenta that correspond to blood-filled spaces at delivery. This entity is not described in the pathology literature. We believe that maternal lakes represent an early stage of intervillous thrombosis and/or perivillous fibrin deposition before the fibrin is laid down. Flow can be demonstrated in some of these lesions at real-time sonography and color flow Doppler examination (Figure 7-20). It is likely that in those lesions with particularly active flow, less fibrin is deposited so that the lesion is "empty" on gross inspection at delivery (Figure 7-21). Aberrant blood flow to the placenta may result in an increased number of maternal lakes, as has been described in cases of placenta accreta (see later discussion).

Infarcts

Placental infarcts result from a disruption or occlusion in the maternal vascular supply leading to a localized area of ischemic villous necrosis. Infarcts occur most commonly at the base of the placenta and vary in size from a few millimeters to many centimeters. Macroscopically, infarcts have a triangular shape with the base of the triangle adjacent to the basal plate.^7,24 Although small infarcts are found in 10% to 25% of placentas of uncomplicated pregnancies, they occur with increased frequency in pregnancies complicated by preeclampsia and essential hypertension.⁷ Small infarcts affecting 5% or less of the parenchyma have little clinical significance in women with healthy parenchyma and adequate placental reserve. Extensive placental infarction is characteristic of an abnormal vascular tree and severely compromised uteroplacental circulation, and is a reflection of maternal vascular diseases such as hyperhomocysteinemia, antiphospholipid syndrome, lupus erythematosus, and other thrombophilic entities.⁷

Placental infarcts cannot be documented at sonography unless they are complicated by hemorrhage.²⁵ The sonographic appearance of placental infarcts changes over time, being hypoechoic initially and becoming more echogenic with age.⁷ This likely reflects the fact that infarcts are composed of necrotic villi, whereas those macroscopic lesions that are visible at sonography contain fluid and/or fibrin.

Maternal floor infarction is a distinct pathologic entity described in the 1960s and portrayed as a marked increase in fibrinoid deposits of the decidual floor (maternal surface) with encasement of villous tips rendering them sclerotic and avascular.^26,27 The etiology of this condition is unknown.²⁷ This entity, in fact, is not an infarct but a "massive basal plate fibrin deposition."⁷ Pregnancy complications including fetal growth restriction, oligohydramnios, preterm birth, and stillbirth have been described in association with this placental lesion and may recur in subsequent pregnancies.^28,29 Owing to the increased risk of recurrence, a thorough examination of the placenta following delivery is prudent. Maternal floor infarction is difficult to diagnose prenatally and may appear as increased echogenicity and thickening along the maternal surface of the placenta and continuing through the placental body in pregnancies complicated by fetal growth restriction and oligohydramnios.²⁹

Gestational Trophoblastic Disease

The gestational trophoblast contributes to the formation of placental villi (Table 7-1). Neoplasms of the gestational trophoblast may be divided into those with villi (complete mole, partial hydatidiform mole, and coexistent mole and fetus) and those without villi (persistent trophoblastic neoplasia, including choriocarcinoma, invasive mole, and placental site trophoblastic tumor).

The complete hydatidiform mole is characterized by replacement of the placenta with enlarged, hydropic villi and the lack of an embryo. It is believed to result from abnormal fertilization of an empty ovum by either a single sperm with duplication of the paternal haploid set of chromosomes (46,XX) or 2 spermatozoa (dispermy), resulting in a heterozygous karyotype (46,XY or 46,XX).^30,31 Complete moles are usually paternally derived, whereas partial moles contain maternal and paternal components and are usually triploid.³²

At sonography, the uterus is filled with echogenic material containing multiple anechoic vesicles (bunch of grapes) of different sizes (Figure 7-22). The vesicles enlarge with advancing gestational age. Early in pregnancy, transvaginal sonography is helpful in diagnosing hydatidiform mole. A viable fetus may coexist with a complete mole in the case of a dichorionic twin pregnancy with one normal fetus and a coexisting complete mole. Elevated maternal serum β-human chorionic gonadotropin (β-hCG) levels are found with hydatidiform mole. Persistent trophoblastic neoplasia may occur in patients with complete hydatidiform moles; therefore, serum β-hCG levels must be followed to ensure that they decrease to zero after evacuation of the mole.³¹

Partial hydatidiform mole refers to a placenta containing areas of molar change alternating with normal villi. The fetus is usually abnormal and exhibits fetal growth restriction and anomalies. Most partial moles are triploid (69 chromosomes). At sonography, a large placenta with diffuse anechoic intraplacental lesions is seen (Figure 7-23). Triploid pregnancies have a high incidence of first trimester spontaneous abortion. In the second trimester, partial moles may present with early-onset preeclampsia. Persistent trophoblastic neoplasia occurs in a small percentage of patients with triploid partial moles, necessitating close monitoring of chorionic gonadotropin levels.^30,31

Primary Neoplasm

Nontrophoblastic primary tumors of the placenta include the relatively common chorioangioma and the rare teratoma. Teratomas are found between the amnion and the chorion usually on the fetal surface of the placenta, and they lack an umbilical cord. They are of no clinical significance.³³ Chorioangiomas are benign vascular malformations seen in approximately 1% of carefully studied placentas. Small tumors occur within the placenta, whereas large tumors may protrude from the fetal surface.

At sonography, a large chorioangioma appears as a well-circumscribed solid, intraplacental, hypoechoic, or mixed echogenic mass lesion within which vessels may be seen (Figure 7-24A,B,C, and D).³⁴ Doppler examination is useful to demonstrate the vascularity of these lesions. Large chorioangiomas (>5 cm) are associated with fetal hydrops, hydramnios, cardiomegaly, and high-output heart failure, low birth weight, premature labor, or fetal demise.^{35,36, and 37} Small lesions usually have no associated problems.

Secondary Neoplasms

Neoplasms that may metastasize to the placenta include melanoma, carcinoma of the breast, and carcinoma of the lung.

The placenta should be easily distinguished from the myometrium and decidua basalis, which appear relatively hypoechoic in comparison (Figure 7-25). Maternal blood flows to the placenta via the spiral arterioles. These terminate at the base of the placenta and supply blood to the intervillous space. Blood drains from the placenta through endometrial veins, which are present all along the base of the placenta,^{38,39, and 40} and in the septa (Figure 7-26). These veins are often visible at sonography (Figure 7-27A and B).

Contractions

Transient myometrial thickening results from normal uterine contractions (Braxton Hicks) that are usually imperceptible to the mother. These contractions most commonly occur in the second trimester, but can be seen earlier (Figure 7-28). A contraction may mimic placenta previa, and the area should be reevaluated at the completion of the sonographic exam to exclude that diagnosis. A contraction should also be distinguished from a leiomyoma, which would not change over a relatively short period of time.

Leiomyomas

Leiomyomas have a variable sonographic appearance ranging from hypoechoic to hyperechoic and complex, depending on their composition and the degree of degeneration. During the course of pregnancy, they may change in size and echotexture. Retroplacental leiomyomas are distinguishable from contractions at sonography by their stable appearance over time; the presence of multiple leiomyomas helps in the diagnosis.

The amnion is seen as a separate membrane (see Figure 7-8) until the end of the first trimester, when the amnion and chorion fuse to form an avascular membrane surrounding the amniotic cavity. In rare cases, the membranes remain separate throughout the pregnancy (Figure 7-29). The yolk sac remnant, which is located between the amnion and the chorion, may be seen at sonography on the fetal surface of the placenta (see Figure 7-8A).

Placenta Extrachorialis

A placenta in which the fetal membranes (chorion laeve) do not insert at the edge of the placenta (maternal villous margin), but attach at some distance inward toward the umbilical cord, is called placenta extrachorialis.^41,42 The chorionic plate is therefore smaller than the basal plate. The attachment of the fetal membranes to the chorionic plate forms a ring that may be flat (circummarginate) or folded (circumvallate) (Figure 7-30). This ring may be partly or totally circumferential. The fold of membranes can be seen at sonography (Figure 7-31) through the second trimester. The circummarginate and partial circumvallate placentas have no clinical significance, whereas a complete circumvallate placenta may be associated with a higher incidence of premature labor, threatened abortion, fetal growth restriction, oligohydramnios, perinatal mortality, and marginal abruption.^41,43

The Umbilical Cord Insertion

The umbilical cord inserts on the fetal surface of the placenta, usually in an eccentric location (Figure 7-32).⁴⁴ A placenta with a marginally inserted cord (within 2 cm of any placental edge) is less common than an eccentrically placed cord and is often referred to as a battledore placenta (Figure 7-33). A marginal cord insertion was previously thought to be associated with poor obstetric outcomes. A recent review of 100 charts to evaluate the impact of prenatally diagnosed marginal placental insertion on birth weight and pregnancy duration was performed. Findings established that prenatally diagnosed marginal cord insertion was not associated with increased risk of growth impairment or preterm delivery.⁴⁵

An important variation of umbilical cord attachment is the rare velamentous insertion, where the fetal vessels insert into the membranes at a variable distance from the fetal surface (see Figure 7-32; Figure 7-34A,B, and C). Velamentous insertion is characterized by umbilical vessels, unprotected by Wharton jelly, that course for a distance between the amnion and chorion before reaching the placental surface.⁴⁴ This is found in approximately 1% of deliveries, but reported frequency varies.^44,46,47 Fetal vessels are less protected and may be damaged during labor. Velamentous insertion is more common in multiple gestations and in gestations with a single umbilical artery.^44,46

A potentially serious complication, vasa previa, occurs when fetal vessels running intramembranously cross the internal os (Figure 7-35). Vessels may tear, leading to significant fetal blood loss and possible exsanguination. Color Doppler examination is helpful to accurately assess an abnormal umbilical cord insertion, especially in patients at risk such as those with bilobed, succenturiate, and low-lying placentas; pregnancies resulting from in vitro fertilization; and multiple pregnancy.^46,48 Thus, some experts will recommend routine Doppler sonography in the lower segment and near the cervical os if patients have these risk factors of vasa previa. As with placenta previa, vasa previa is an indication for cesarean delivery. These conditions, abnormal placental cord insertions and vasa previa, can be explained by the concept of placental trophotropism. Trophotropism describes a process of focal enlargement or atrophy of the placenta probably dependent on several factors that determine relative myometrial perfusion.

At the end of the fourth month, the placenta has attained its final thickness and shape; however, circumferential enlargement continues into the third trimester. The appearance of the placenta and myometrium may change with contractions (see earlier discussion). It may also change with different degrees of bladder filling; this effect occurs most commonly in the second trimester. The full bladder compresses the anterior portion of the lower uterine segment against the posterior wall, artificially elongating the cervix, and may give the appearance of a low placental location. It may be necessary to repeat the examination after the patient voids, to better define the lower uterine segment and cervix, and the possibility of placenta previa. (Figure 7-36).

Succenturiate Lobes

In approximately 5% to 6% of routinely examined placentas, succenturiate (accessory) lobes are present.^41,49 These consist of separate masses of chorionic villi connected to the main placenta by vessels within the membranes (Figure 7-37). It is important to make the diagnosis prenatally because of the following complications: a succenturiate lobe may be retained in utero, resulting in postpartum hemorrhage; it may overlie the internal cervical os; or the vessels that connect it to the main placental mass may traverse the internal os (vasa previa) and rupture during labor, resulting in fetal blood loss. Color Doppler is a valuable adjunct to identify and follow the course of vessels connecting the accessory lobe to the main placental body.

Placenta Membranacea

Other variations of placental shape that are of clinical significance include placenta membranacea and partial placenta membranacea.^42,50 Both are associated with antepartum and postpartum hemorrhage, miscarriage, and prematurity. Bleeding may begin early in pregnancy and likely occurs because a placenta membranacea is essentially a placenta previa. These rare conditions result from failure of the normal process of atrophy of the chorionic villi in the first trimester, so that villi cover the entire outer surface of the amniotic sac.⁴² Early in gestation, the entire sac is covered by villous chorion. As the pregnancy progresses, villi oriented toward the uterine cavity (covered by decidua capsularis) atrophy and the chorion laeve is formed.⁴² Placenta membranacea may be diagnosed at sonography if placental tissue is seen to extend over most of the uterine cavity.⁵¹

Bleeding is not uncommon in pregnancy. Blood loss from placenta previa and abruptio placentae usually occurs at or close to term, whereas retroplacental or marginal hemorrhage may occur as early as the first trimester.

Retroplacental/Submembranous Hematoma

Retroplacental hemorrhage may manifest itself in 3 ways: (1) external bleeding without formation of a significant intrauterine hematoma; (2) formation of a retroplacental or marginal hematoma with or without external bleeding; and (3) formation of a submembranous hematoma at a distance from the placenta, with or without external bleeding. Hematoma and thrombi may occur at various locations in or around the placenta (Figure 7-38). Sonographic examination in cases of antepartum bleeding may be negative if most of the bleeding is external.

A retroplacental or submembranous hematoma will appear as a hypoechoic or complex collection at sonography.^{52,53, and 54} At least one follow-up examination should be performed to ensure that the hematoma has decreased in size. Even if follow-up study indicates that the lesion has disappeared, careful examination of the placenta following delivery will show a thin layer of organized hematoma or fibrin along the membranes.

In early pregnancy, subchorionic hematoma may be distinguished from a nonfused amnion by evaluating the thickness of the membrane. The membrane above a subchorionic hematoma is thicker than the amnion (see Figure 7-7 and 7-39).

The clinical significance of retroplacental hemorrhage depends on the size and extent of the lesion. Small hematomas are not associated with an increased risk of pregnancy loss and are of little significance.^{7,55,56, and 57}

Abruptio Placentae

The term abruptio placentae is usually reserved for the clinical syndrome of acute separation of a normally implanted placenta. In extreme cases, this may be accompanied by severe bleeding, pain, and hypovolemic shock. The location of the hematoma formation is between the basal plate (maternal surface) of the placenta and the uterine wall, separating the two.⁷ The incidence of this condition has steadily increased, and a clinical placental abruption is estimated to occur in 0.5% to 1% of pregnancies. This increased trend may represent a true increase in abruption or improvement in diagnosis.^7,58 In stable patients, a poorly defined, inhomogeneous echogenic retroplacental collection causing apparent thickening of the placenta may be seen between the placenta and myometrium at sonography. The absence of sonographic findings does not exclude the diagnosis of placental abruption. Traditionally, abruptio placentae have been associated with maternal hypertension, preeclampsia, preterm premature rupture of the membranes, cigarette smoking, and cocaine use.^7,59,60 Increased vascular resistance in the uteroplacental arteries has been documented in association with abruptio placentae in hypertensive patients.^61,62

Placenta Previa

Placenta previa is diagnosed when the placental implantation covers part or all of the internal cervical os and is a common cause of bleeding in the second and third trimester of pregnancy (Figure 7-40).⁶³ Placenta previa is often overdiagnosed in the first 2 trimesters, because it is commonly mimicked by 2 conditions: (1) an overdistended urinary bladder that compresses the anterior lower uterine segment and posterior lower uterine segment giving the appearance of a previa, and (2) focal contractions (see earlier discussion).⁶⁴ Thus, if placenta previa is suspected, it is important to reexamine the patient after voiding and/or after approximately 20 or 30 minutes have passed. Transabdominal, transvaginal, and transperineal scanning techniques have been utilized to confirm placental location.

Four locations describe the implantation of the placenta in relationship to the internal cervical os⁶³:

1. Low-lying placenta: A low-lying placenta is one in which the lower placental edge is within 2 cm of the internal cervical os, or describes a low implantation in the second trimester.⁶⁵

2. Marginal placenta previa: With marginal previas, the inferior placental margin abuts the internal cervical os.

3. Partial placenta previa: With partial previas, the inferior placental margin extends partially over the internal cervical os.

4. Complete placenta previa: With complete previas, the placenta completely covers the internal cervical os.

It is oftentimes difficult to distinguish sonographically between a marginal and partial placenta previa. Early in gestation, the placenta may appear low-lying or to cover the internal os. Later in gestation, the placental location appears to have migrated. This may be due to placental modeling or differential growth of the lower uterine segment as the pregnancy progresses. It is thought that the placenta grows toward well-vascularized decidua in the fundus, and portions of the placenta overlying the less-well-vascularized cervix undergo atrophy.⁶³ This concept of placental modeling is known as trophotropism.⁶³ Atrophy may leave vessels running through the membranes (vasa previa), or with incomplete atrophy, a succenturiate lobe may be found.⁶³

A recent study quoted the prevalence rate of placenta previa to be approximately 4 in 1000 births.⁶⁶ Risk factors for placenta previa include advanced maternal age, increasing parity, increasing number of prior cesarean deliveries or uterine surgery, increasing number of uterine curettages, male fetus, smoking, cocaine use, and multiple gestations.^63,66,67

Anatomic depiction of the umbilical cord is possible by ultrasound and is part of most anatomic surveys in the second and third trimesters. Abnormal umbilical cord morphology may be a variant of normal or may be associated with fetal karyotypic abnormalities, genetic syndromes, structural fetal anomalies, or untoward pregnancy complications such as fetal growth delay. Additionally, Doppler flow studies of the umbilical artery for assessment of vascular impedance in complicated pregnancies are an important diagnostic tool.

Formation of the Umbilical Cord

The primitive umbilical ring represents an oval line of reflection between the amnion and embryonic ectoderm.⁶⁸ During the fifth developmental week, the connecting stalk (containing the umbilical vessels and allantois) and the yolk stalk (containing the vitelline vessels and the original attachment of the yolk sac) pass through the ring.⁶⁸ The yolk sac is then located between the amnion and chorion. With rapid growth of the amniotic cavity, the amnion encases the connecting stalk and yolk stalk, which coalescence into the umbilical cord.⁶⁸ The yolk sac, located in the chorionic cavity, is connected to the umbilical cord by its stalk. As the amnion grows to meet the chorion, the yolk sac gradually recedes.⁶⁸ The embryonic allantoic arteries form the umbilical arteries. The left allantoic vein develops into the umbilical vein, whereas the right allantoic vein regresses.⁴⁴ The omphalomesenteric vessels are obliterated. The result is an umbilical cord, covered by amnion and containing a single umbilical vein, and 2 umbilical arteries supported in Wharton jelly, a mesodermally derived mucoid matrix.⁴⁴ The cord itself is devoid of lymphatic vessels and nerves.⁴⁴ Although the walls of the umbilical vessels have a large proportion of muscle, they lack collagen and elastin and so are able to change configuration with changes in osmotic pressure in the amniotic fluid.

Structure and Function of the Umbilical Cord

The umbilical cord has a variable length, and normally contains 2 umbilical arteries and a single large umbilical vein surrounded by a clear mucoid connective tissue, Wharton jelly. Wharton jelly surrounding the vessels apparently has the function of protecting the vessels from undue torsion and compression.⁶⁹ A layer of amnion covers the umbilical cord except near the fetal insertion, where an epithelial covering is substituted. The arteries wind around the umbilical vein in a spiral fashion because the umbilical vessels are longer than the cord.⁶ There may be a number of foldings, loops, and tortuosities producing protrusions or false knots on the cord surface. Both the helical pattern (coiling) of the cord vessels and the presence of Wharton jelly confer turgor to the cord, which helps prevent kinking and cord prolapse. In approximately 1% of pregnancies, true knots are found, which may be an incidental finding or can be associated with untoward pregnancy outcomes.⁶

Oxygenated blood flows in the umbilical vein from the placenta to the fetus and, on reaching the fetal abdominal wall, passes through the liver posteriorly and cephalad to terminate at the portal sinus (the main left portal vein). Deoxygenated blood from the fetal aorta passes to the hypogastric arteries, which wind superiorly and medially along the superolateral margin of the bladder, to enter the cord as the umbilical arteries, which carry blood back to the placenta.

SONOGRAPHIC ANATOMY OF THE NORMAL CORD

The umbilical stalk and the yolk sac are seen as early as 7 weeks adjacent to the anterior abdominal wall of the developing fetus. The yolk sac is extra-amniotic (Figure 7-41). In the late first trimester and in the second and third trimesters, the umbilical cord is readily visualized. As imaged in long axis, the cord may be seen as a series of parallel lines and shorter angled, linear interfaces arising from the umbilical arteries that wrap around the central vein (Figure 7-42A and B). In a cross-sectional view, the arteries and umbilical vein may be seen as 3 separate circular lucencies (Figure 7-43). Pulsations of the cord, occurring at the same rate as fetal heart rate, may be seen in real time.

Because there are different directions of flow in the umbilical cord, a color Doppler image will demonstrate one color in the vein and another in the arteries (see Figure 7-42B). The umbilical arteries course laterally around the fetal urinary bladder and may be identified with either color or power Doppler (Figure 7-44). The cord insertion into the placenta may appear as a V- or U-shaped sonolucent area arising from the chorionic plate.

CORD ABNORMALITIES

Eccentric or marginal cord insertions, velamentous insertions, and vasa previa were mentioned earlier in this chapter. At the insertion of the cord into the anterior abdominal wall of the fetus (fetal insertion), the origins of the umbilical vein and hypogastric arteries may be seen. Weakness in the anterior abdominal wall of the involuted right umbilical vein is one etiology for gastroschisis, a full-thickness abdominal wall defect with an eccentric herniation of bowel.

Persistent right umbilical vein is identified sonographically by (1) the umbilical vein being connected to the right portal vein rather than the left; (2) the position of the intrahepatic portion of the umbilical vein being lateral to the gall bladder, rather than medial; and (3) the portal vein curving toward the stomach rather than toward the liver.⁷⁰ The incidence of persistent right umbilical vein is approximately 0.2% and is typically an isolated finding. If no additional structural anomalies are detected, this condition represents a benign variant with a favorable prognosis.^71,72

Abnormalities of Cord Length

The average normal umbilical cord length is about 55 cm but ranges from 30 to 90 cm in length and is 1 to 2 cm in diameter.^6,44,46 Extremes of cord length may occur from apparently no cord (achordia) to lengths of up to 300 cm.⁷³ Achordia has been associated with abdominal wall/ limb-body wall defects.⁴⁶ Excessively long cords may predispose to vascular occlusion by thrombi, true knots, entanglement, and also to cord prolapse after membrane rupture. Excessively short umbilical cords have a length of 32 cm or less.⁴⁴ Short cords are associated with congenital anomalies, intrapartum fetal distress, and fetal death.⁷⁴ Uterine inversion and placental abruption may occur as a result of a short umbilical cord. Childhood mental and motor impairment, and depressed intelligence quotient values are correlated with excessively short cords.^44,46 It is uncertain whether the length of the umbilical cord is the cause or the result of an underlying central nervous system problem.^44,46

Sonographic delineation of the umbilical cord with color or power Doppler sonography may be helpful in a qualification assessment of cord length. Thus, a subjective assessment of cord length is possible (see Figure 7-42).

Abnormalities of Cord Position

Loops of umbilical cord usually lie anterior to the fetal abdominal wall and adjacent to the limbs. In a number of instances, however, loops of cord may encircle the fetal neck or limbs, or loops of cord may lie between the fetal presenting part and the lower uterine segment (funic presentation).

Nuchal Cord

A single nuchal cord is common in pregnancy and is reported in 25% to 30% of pregnancies (Figure 7-45). Nuchal cords are generally not associated with significant fetal compromise or complications.⁷⁵ However, there have been pregnancy complications reported in association with nuchal cords such as a nonreassuring fetal heart rate tracing, meconium staining, 5-minute Apgar score less than 7, assisted ventilation, and cerebral palsy.⁷⁵ Multiple loops are seen in only 2% of pregnancies.

Funic Presentation

Occasionally, loops of the cord may be seen lying between the fetal presenting part and the lower segment, referred to as cord or funic presentation. It is important to recognize this because such a position predisposes to cord prolapse and possible fetal death at the time of rupture of the membranes. Funic presentation is more common with malpresentations, such as breech or transverse lie. The condition, especially before 32 weeks, may often be transient and clinically insignificant; however, one should look for causes that may produce a persistent cord presentation and risk cord prolapse. These causes include a marginal cord insertion from the caudal margin of low-lying placenta, uterine structural anomalies such as fibroids or uterine adhesions, or congenital malformations that may prevent the fetus from engaging well into the lower uterine segment.

Single Umbilical Artery

Although the normal umbilical cord contains 2 umbilical arteries, a single umbilical artery (SUA) may be seen in approximately 1% of all newborn singletons and occurs more frequently in twins and abortuses (Figure 7-46A,B,C, and D).^46,76 Heifetz in an extensive review found the prevalence of SUA to be 0.63%.⁷⁷ SUA occurs more frequently in women with diabetes, epilepsy, preeclampsia, antepartum hemorrhage, hydramnios, and decreased amniotic fluid.⁷⁸ Fetal malformations are often found with SUA with an incidence that varies in the literature at a range between 25% and 50%, and with an increased frequency of renal and cardiac anomalies, fetal growth restriction, and karyotypic abnormalities.^44,79,80

A single umbilical artery may be the consequence of aplasia or of atrophy of one artery.⁴⁶ Routine ultrasound screening is a reliable tool to identify umbilical vessels either as a free loop or with color or power Doppler at the level of the fetal urinary bladder. Caution should be exercised when evaluating the cord vessels close to the chorionic plate as the 2 arteries may fuse into a single trunk near the cord insertion into the placenta.^44,46

To summarize the discussion of SUA, the following points are made:

1. There is a positive association of fetal anomalies with SUA involving any organ system.

2. If SUA is associated with a detected fetal anomaly, there is a high risk of aneuploidy.

3. A fetus with SUA and no detectible anomaly is still at risk (although small) for a clinical abnormality, and clinical judgment needs to guide decisions as to whether or not to pursue invasive prenatal diagnosis for karyotyping.

4. Even if isolated, pregnancies with a single umbilical artery are at increased risk for fetal growth delay, and serial sonography may be considered.

True Cord Cysts

True cord cysts may be due to allantoic or omphalomesenteric duct remnants, whereas false cysts result from focal liquefaction of Wharton jelly, often occurring within a regional zone of thickening of the jelly. All are very uncommon in the second and third trimesters. Cysts can be seen more frequently, although they are still uncommon, in the first trimester. Most spontaneously resolve by the end of the first trimester. Such cysts, even when large, usually do not jeopardize fetal circulation, but isolated case reports of cord accidents from cysts and other focal cord lesions indicate the need for ongoing monitoring for fetuses that have any cord lesion. In pregnancies where multiple cysts are identified or cysts identified in the first trimester persist are at increased risk for structural and karyotypic abnormalities.⁴⁴ Sachs et al⁸¹ reported the prenatal diagnosis of a 5-cm cystic mass within the umbilical cord several centimeters from the abdominal wall at 21 weeks of gestation. Visualization of vessels in the lateral wall of the mass and an intact anterior abdominal wall allowed exclusion of other pathologies.

Other

True knots may be seen in pregnancies with excessively long umbilical cords and in pregnancies complicated by hydramnios (Figure 7-47). They are formed by the fetus moving through a portion of cord. The incidence of true knots varies in the literature with an incidence ranging from 0.04% to 1.5%.⁴⁴ True knots are identified more frequently in monoamniotic twin gestations due to the lack of a dividing membrane. False knots of the umbilical cord represent vascular redundancies of the umbilical vessels. False knots are of little clinical significance.⁴⁶

Hematomas of the umbilical cord may be the result of amniocentesis, fetal blood sampling, or rupture of a varix.⁴⁴ Cord hematomas are occasionally seen with short cords, entangling, and trauma.⁴⁶ In many instances of cord hematoma, no etiology is elucidated. Thrombosis of umbilical vessels may accompany cord compression, torsion, stricture, or hematoma formation. These usually occur in the umbilical vein.⁴⁴ Other neoplasms of the umbilical cord, which are quite rare, include angiomyxomas, myxosarcomas, and choriomyxomas, and in fact probably represent hemangiomas. Hemangiomas are rare and described as rounded or ovoid swellings in the cord.⁴⁴ Sonographically, hemangiomas may appear as echogenic masses related to the cord.⁸² They may be associated with elevated α-fetoprotein values. Teratomas represent primary tumors of the cord with cystic and solid components.⁴⁴

Umbilical hernia is one of the most commonly encountered abnormalities in early infancy.⁸³ Umbilical hernia has been reported as being more common in trisomy 21, congenital hypothyroidism, mucopolysaccharidoses, and the Beckwith syndrome.⁸³ Sonographically, umbilical hernia may be recognized as a protrusion from the anterior abdominal wall, with a normal insertion of the umbilical vessels.

Omphalocele and gastroschisis represent abnormalities of closure of the anterior abdominal wall. With omphalocele there is a midline umbilical defect with protrusion of abdominal structures, such as bowel and liver, into the base of the umbilical cord. This produces a sonographic appearance of a mass adjacent to the anterior abdominal wall, covered with a membrane, and into the apex of which the umbilical cord appears to insert. There is a high incidence of other anomalies (eg, intestinal, cardiac, and renal anomalies). Recent studies have shown a difference in the risk of aneuploidy in fetuses with small omphaloceles that tend to contain bowel only, whereas large ones contain both bowel and liver.

With gastroschisis, a right paraumbilical abdominal wall defect results in protrusion of bowel and other intraabdominal contents into the amniotic fluid. The cord normally inserts just to the left of the defect; therefore, gastroschisis will appear as a complex mass adjacent to the base of the cord. The exteriorized bowel loops are not covered by a membrane, and therefore are directly exposed to amniotic fluid. A more detailed discussion of the entities can be found in Chapter 17 on sonography of fetal gastrointestinal tract anomalies.

Recent studies have suggested that fetuses with a noncoiled umbilical cord are at greater risk for karyotypic abnormalities and poor fetal conditions when compared with controls.⁸⁴ The detection of a coiled cord can be enhanced with the use of color Doppler sonography to show the "pitch" (number of coils per unit length) of umbilical arteries as they course around the umbilical vein. Further studies devised a coiling index of the cord by dividing the number of complete coils by the length of the cord. Fetuses with low coiling indices had poorer outcomes.⁸⁵ Overcoiling or undercoiling of the cord is reported in association with poorer perinatal outcomes.⁴⁴

Amniotic band syndrome is thought to be a sequela of rupture of the amnion, with formation of fibrous bands that may cross fetal parts and cause significant maldevelopment (Figure 7-48A).⁸⁶ The entrapped fetal part typically is entangled within the area of the membrane. Amniotic band syndrome has been associated with craniofacial and spinal malformations (anencephaly, exencephaly, facial clefts), gastroschisis, and limb/extremity amputation.^87,88

A thin interface within the uterus may be encountered in patients in whom the amniotic membrane is draped over a uterine synechia. This uterine malformation has been termed an amniotic sheet and has not been associated with an increased incidence of fetal malformation as its location is extra-amniotic (Figure 7-48B).^89,90 Amniotic sheets are broad based and composed of 4 layers, 2 amniotic and 2 chorionic. Their average thickness is 2.4 mm in the midportion and 4.5 mm at the free edge. Women with amniotic sheets have an increased incidence of previous uterine surgery (uterine curettage, cesarean) and uterine infections (endometritis).^{91,92, and 93}

Uterine septa tend to be thicker than the interfaces created by amnion and chorion. The septae may consist of myometrium, a fibrous septation, or both, which typically have a (mid) sagittal orientation within the uterus. Implantation can occur on the septum and may account for a higher incidence of spontaneous abortion.⁹⁴ Placentation on a septum predisposes to abruption, fetal growth restriction, and placental adherence to the septum. There does not seem to be any increased incidence of complications in patients in whom the placental insertion is on the amniotic sheet.⁹⁵

It is important to document the presence of a membrane between fetuses and the type of placentation when evaluating a multifetal pregnancy. The amnion and chorion of dichorionic twin pregnancies tend to form a triangular configuration that projects into the amnion, termed a twin peak.⁹⁶ Determination of membranes is most accurate in the first trimester; thus, with diagnosis of multiple gestation, the membranes should be assessed for the "twin peak" sign of dichorionic or the "T sign" of monochorionic membranes. Prenatal sonographic determination of the makeup of the membrane is important because dichorionic diamniotic gestations have the best prognosis, and monochorionic monoamniotic gestations have the poorest. In monochorionic diamniotic placentation, shared placental vasculature may result in fetal growth and amniotic fluid discordance, and in some cases, twin transfusion syndrome. Intertwining of umbilical cords, conjoined twins, or more than 3 vessels in the umbilical cord occurs only with monochorionic monoamniotic twinning.

This chapter described the sonographic depiction of the placenta, umbilical cord, and intrauterine membranes. Although disorders of these structures occur relatively rarely, sonographic depiction of the type and extent of the disorder can aid management of complicated pregnancies. Placental location and adherence plays an important role in clinical obstetrics.

Acknowledgments

The authors thank Drs Beverly A. Spirt, Lawrence P. Gordon, and Arthur C. Fleischer for their initial contribution to this chapter in previous editions.