Chapter 30: POSTPARTUM ULTRASOUND

During pregnancy, numerous dramatic physiologic changes take place, and then after delivery, whether by the vaginal or cesarean route, these changes resolve even more rapidly. This process of resolution occurs during the puerperium, which is the 6 weeks following delivery. In 1972, Robinson first described using diagnostic ultrasound to image the postpartum pelvic organs.¹ Since that time, ultrasound has continued to play an important role in the diagnosis of postpartum anatomical structures, particularly in conditions of infection, hemorrhage, or suspected retained placenta; processes that require imaging include assessment of women with persistent postpartum endomyometritis, pelvic abscesses, retained placental fragments, and hematomas of the uterus, bladder, peritoneal flap, or abdominal wall. Such complications can involve up to 5% to 10% of postpartum women.²

This chapter reviews normal and pathologic sonographic findings during the puerperium, with the major emphasis on the pelvic organs, although the urinary tract, hepatobiliary tract, and vasculature are also covered. The normal anatomy, including variations of normal, and the changes associated with pregnancy and further alterations by the various disorders are reviewed.

Normal Anatomy

The Uterus

In the nonpregnant state, the uterus occupies a midline position that measures approximately 8 cm in length. Uterine size may be affected by parity, and pathologic conditions such as leiomyomata or adenomyosis.³ The uterus grows from a pregravid weight of 140 g to a peak of 1 kg at term; the uterine blood flow increases from 50 mL/min to 500 mL/min at term.² The uterus shrinks remarkably during the puerperium, without cellular destruction; instead, there is a reduction in cell size with a simultaneous loss and resorption of tissue fluid and contractile proteins. Similarly, the animal model shows an orderly process of cellular restitution with cytoplasmic and collagen disintegration without tissue necrosis. An awareness of the expected uterine size is essential to the interpretation of the ultrasound findings (Figure 30-1). The most rapid proportion of uterine involution occurs during the first 2 weeks postpartum with a nearly 50% reduction of uterine size.^{4,5,6, and 7} Wachsberg et al⁶ have demonstrated that the postpartum uterus approaches the nongravid state within 6 to 8 weeks following delivery. These investigators also noted that uterine contractions and bladder size may affect the measurements. Route of delivery does not affect the postpartum uterine dimensions.

The postpartum uterus involutes rapidly:⁴

• At 24 hours: L 17.5 cm × W 12.3 cm × anteroposterior (AP) 9.0 cm; endometrial AP cavity = 1.2 cm

• 1 week: L 12.9 cm × W 11.3 cm × AP 8.7 cm; endometrial AP cavity = 1.3 cm

• 2 weeks: L 11.0 cm × W 7.7 cm × AP 7.8 cm; endometrial AP cavity = 1.0 cm

The postpartum myometrium shows a heterogeneous echo appearance related to the pregnancy-related changes: increased vascularity, and resolution of edema and fluid content. The vascularity usually resolves relatively quickly such that by 3 to 4 weeks postdelivery (see Figure 30-2), the vascular channels should have the pregravid appearance.⁶ Uterine fibroids may continue to disturb the usually quiescent appearance of the heterogeneity.

The endometrial cavity maintains a consistent measurement of less than 2 cm in the anteroposterior dimension during the early puerperium. Variations of normal may be noted. For instance, a small amount of fluid may be seen in many normal postpartum uteri, and should not be connoted to be pathologic. Furthermore, the endometrial–myometrial interface may be variable; it may be smooth and well defined, or irregular and heterogeneous. One of the most important roles of sonography of the postpartum uterus is to identify retained placenta or products of conception. An enlarged endometrial cavity (AP diameter >2.5 cm on transabdominal sonography) is often associated with a hypotonic uterus or retained products. It is often difficult to discern blood clots from placenta fragments; however, an echogenic mass within the uterine cavity in the face of secondary postpartum hemorrhage is very strong evidence for retained products.^9,10 Deans and Dietz¹¹ studied 94 postpartum women prospectively, and correlated normal uterine findings with clinical course and subdivided their sonographic findings to the upper and lower uterus. The upper uterine segment area had an average thickness of 13.8 mm and an average volume of 35.6 cm³, and the lower segment/cervical area held considerably more material with an average volume of 54.8 cm³. The overall endometrial echo appearance will differ depending on the uterine size and postpartum state.

Vulva, Vagina, and Pararectal Space

The vagina is a hollow muscular tube that extends from the vulvar vestibule to the uterus. Whereas the lower third of the vagina is closely opposed to the urogenital diaphragm and pelvic diagram, the upper and middle thirds of the vagina are supported by the levator ani and the cardinal ligaments. The vagina undergoes marked distension during the delivery process. Lacerations to the vagina, or damage to blood vessels adjacent to the vagina can occur. In women who have undergone an instrumented vaginal delivery, or those patients with a coagulation defect such as associated with placental abruption, vulvar, or vaginal hematomas are not uncommon. Ultrasound may help to delineate a vaginal hematoma, but magnetic resonance imaging (MRI) is probably superior in assessing possible extension into the retroperitoneal space.¹¹ Also, in general, the clinical picture will dictate the management.

Similarly, hematomas of the vulva or distal vagina may tract into the paravaginal or pararectal space. In this manner, women may lose significant blood into these potential spaces without external manifestation of hemorrhage or outward hematoma. Imaging with computed tomography (CT) or MR is superior to ultrasound for assessment of these conditions due to the nature of the anatomical space, and the tenderness of genital tract hematomas.¹¹

The Broad Ligament

The broad ligaments are double reflections of the parietal peritoneum extending from the bony pelvic side walls to the uterus. It envelopes the uterine vasculature, the fallopian tubes, and to some extent, the ovaries. In the normal patient, the broad ligament is not easily visualized sonographically. Nevertheless, pathological conditions within the broad ligament such as hematoma, abscess, fluid collections, or fibroids may be easily seen (Figure 30-3).

The Ovaries

In the nonpregnant woman, the ovaries are usually found near the ovarian fossa, which are depressions of the peritoneum adjacent to the external iliac vessels. During the puerperium, the ovaries are lifted by the utero-ovarian ligaments above the true pelvis, and can be demonstrated sonographically about half the time (Figure 30-4).⁴ During the first trimester, ovarian cysts such as corpus lutea are often seen; after this time, these cysts usually regress. Not infrequently, luteomas of pregnancy are seen, usually noted as multicystic ovaries, often bilateral, with thin septations. Luteomas are physiologic changes of the ovaries in response to the human chorionic gonadotropic (hCG) hormone, and may lead to the production of androgens. Hence, luteomas of pregnancy may be seen in patients with conditions of markedly elevated hCG levels such as with molar pregnancies or multiple gestations. These changes in the ovaries almost invariably resolve with resolution of the pregnancy, although they can persist for months; unwarranted surgery may lead to profuse hemorrhage. Their bilateral thin-walled appearance is typical. Rarely maternal or female fetal virilization may be seen.¹²

The Cul-de-sac

The cul-de-sac is the space between the uterus and the rectum and is the most dependent portion of the pelvis. Ascitic fluid or blood may settle in this region. In the absence of free fluid, the cul-de-sac is difficult to visualize sonographically. A small amount of fluid or blood following delivery is normally present.

Various extraperitoneal spaces are usually not seen sonographically. These compressed potential cavities include the retropubic space of Retzius, and pararectal, paravesical, and paravaginal spaces where blood or fluid may possibly accumulate. Operative vaginal delivery, maternal coagulopathy, cesarean delivery, and rarely spontaneous hemorrhage may lead to hematomas in these regions (Figure 30-5). In conditions of suspected retroperitoneal pathology, CT or MR imaging is usually superior to ultrasound.¹³

Pathology

Postpartum Hemorrhage

Postpartum hemorrhage (PPH) is typically divided into early (<24 hours after delivery) and late (>24 hours). Early PPH is usually due to uterine atony, genital tract lacerations, coagulopathy, or retained placental fragments. Often, clinical circumstances such as a boggy uterus hints toward the etiology in early PPH, and the management is directed without ultrasound. However, the concomitant use of ultrasound can be useful as an adjunct to curettage of the boggy uterus because the usual palpable definition of the uterine cavity may be absent to the operator. The postpartum uterus is easy to perforate.²

Late postpartum hemorrhage is seen in less than 1% of postpartum women, and usually is related to either subinvolution of the placental implantation site or retained placental tissue (Figure 30-6). One of the most difficult diagnostic challenges to the sonologist is distinguishing retained placental fragments versus uterine blood clots. Blood clots tend to be homogeneous, whereas placental fragments tend to be more echogenic; however, this is not universal. This is an important differentiation, in that retained placental fragments left in situ can lead to further hemorrhage or infection; in contrast, postpartum curettage will increase the likelihood of damage to the decidual basalis layer leading to uterine synechiae and infertility.¹⁵ Sonography has been shown to be helpful in identifying products of conception after spontaneous abortion.¹⁵ Dewhurst¹⁶ demonstrated that less than one-third of 89 patients who underwent dilatation and curettage (D&C) for PPH had pathologically confirmed choronic villi. Placenta accreta can be a contributing factor to retained products.^17,18

Hertzberg and Bowie²¹ suggested 5 categories of diagnosis when assessing the puerperal uterus for retained products with ultrasound (Figure 30-6). In their series of 53 patients, the finding of an echogenic mass was the most common finding associated with retained placenta. When a thin endometrial stripe or only endometrial fluid was seen, they found no patients with retained products on follow-up pathological examination or clinical follow-up. Their 5 categories include:¹⁹

Postpartum Infection

Puerperal infection may occur in approximately 1% of vaginal and up to 29% of cesarean deliveries, which has been noted to decrease to 2% to 6% when antibiotic prophylaxis is given.²⁰ Factors that place the woman at risk are listed in Table 30-1. The organisms are typically bacteria, which are vaginal flora that ascend into the uterus. In cesareans, anaerobic bacteria are particularly prevalent.

Endomyometritis is a clinical diagnosis in which affected patients have fever, uterine tenderness, elevated leukocyte count, and possibly foul-smelling lochia. This condition is a diagnosis of exclusion of other sources of fever and usually responds to broad-spectrum antibiotic therapy. In most cases, sonography of the uterus will not be particularly abnormal. However, certain ultrasound characteristics support the diagnosis of endomyometritis, such as a dilated or irregular endometrial cavity. Shadowing echoes within the uterus that may be caused by gas bubbles in the uterine cavity in the absence of instrumentation has been described but is rarely seen. Also, fluid in the cul-de-sac may be noted. Retained placental fragments or blood in the uterus may be a nidus for infection, particularly in those patients presenting remote from delivery. Women with postpartum fever with suspected uterine etiology or without a discernible cause are usually started on broad-spectrum intravenous antibiotics. Imaging is not usually performed. An exception is when patients may present with concomitant anemia with the possible hematoma, or where a suspected pelvic mass is noted.^21.22

One of the main utilities of ultrasound is to evaluate the postpartum patient with persistent fever after adequate antibiotic therapy of 48 to 72 hours duration. In these instances, imaging the uterus and pelvis with the goal of identifying phlegmon, abscess, hematoma, or septic pelvic thrombophlebitis is often undertaken.^23.24 Absence of pathology is reassuring, whereas the identification of a mass often necessitates further imaging such as with CT or MR for delineation.^25.26

Cesarean Delivery

Cesarean deliveries now comprise over 31% of deliveries in the United States.²⁷ The movement away from vaginal birth after cesarean, and the advent of primary cesarean on demand means increasing primary and repeat cesareans. Ultrasound has a special role for patients who undergo cesarean. The involution of the uterus is identical regardless of route of delivery.⁴ Because of the surgical nature of the cesarean, hemorrhage and collections of hematomas sometimes form along the operative dissection planes. A systematic imaging approach should be undertaken: bleeding in the subcutaneous tissue (this is usually clinically apparent) and hematomas under the fascia with bleeding from the rectus muscle are readily visualized by ultrasound and may be especially seen with the low transverse (Pfannenstiel) incisions. The anterior peritoneal reflection between the uterus and the bladder flap can be a source of hematomas. Lastly, hemorrhagic collections of the uterine incision are a source of concern particularly because they can become infected. The sonographic appearance of a hematoma is heterogeneous thickening. Often the omentum may overlie the hemorrhagic hysterotomy incision leading to fever, pain, and a complex heterogeneous mass anterior to the uterus. Vertical hysterotomy incisions are at increased risk for hemorrhagic complications and the sonographic findings may be near the uterine fundus rather than the lower segment. Free peritoneal fluid may be seen with frank bleeding such as from the uterine artery at the lateral aspect of the hysterotomy incision. Figure 30-7 shows a placenta within the peritoneal cavity indicative of an abdominal pregnancy.

Edematous changes postsurgery can be normal, and should not be confused with infection or blood.

• Subcutaneous hematoma: irregular heterogeneous mass in subcutaneous tissue

• Subfascial hematoma: hyperechoic heterogeneous mass below the fascia, above the peritoneum

• Bladder flap: irregular heterogeneous thickening between the uterus and bladder

• Uterine incision hematoma: complex mass depending on the organization, infection, and edema process; anechoic areas (sonolucency) exceeding 2 cm are suspicious for hematoma, or echogenic mass in the lower segment of the uterus

Post-cesarean uterine incision wound healing has been investigated using sonography. Burger et al²⁶ reviewed the sonographic appearance of 48 puerperal women. Although different patterns of thickening and fluid-filled cystic areas were described, no correlation could be established between sonographic findings and long-term clinical outcome such as healing, infection, or uterine dehiscence (Figures 30-8 and 30-9).

Michaels et al²⁸ noted sonographic changes in the antepartum uterus including thinning of the lower uterine segment (<5 mm), ballooning, and wedge defects. Although the study was not designed to investigate the association between sonographic findings and labor outcomes, there seems to be a relationship between some of the antepartum findings and labor management.^{29,30,31, and 32}

Transperineal sonography has been shown to be a useful adjunct to transabdominal examination for identification of postoperative complications such as hematoma or abscess. It can be particularly useful in examining the lower segment of the uterus since the transducer may be painful due to the recent surgery.³³

Because of the marked increase in the intravascular volume and cardiac output, the renal blood flow, glomerular filtration rate, and urine output are increased. Hormonal factors and uterine obstructive factors also lead to ureteral dilation and physiologic mild hydronephrosis. Peake et al prospectively studied 159 pregnant women and found hydronephrosis developing as early as 11 to 15 weeks' gestation, prior to the uterus being a physical reason for obstruction.³⁴ Two-thirds of subjects were affected. Pregnancy-related hydronephrosis seemed to peak at 24 to 28 weeks' gestation with the maternal renal pelvis approximately 63% larger than nonpregnant controls. Sonographic assessment of the maternal urinary tract is useful, since radiologic procedures are generally avoided in pregnant women if possible. An initial screening sonogram of the kidneys and ureters is useful in evaluation for possible nephrolithiasis; however, for definitive diagnosis, CT or selected intravenous pyelography (IVP) is more useful.

Fetal sonographic evaluation may lead to an in utero diagnosis of congenital urinary abnormalities such as renal agenesis or dysgenesis.^35,36 Because of the high (9%) association of renal abnormalities in the parents of affected infants, ultrasound is a valuable screening tool in assessment of the maternal anatomy. Some investigators have recommended that all first-degree relatives of affected fetuses be screened for asymptomatic malformations when such fetal renal anomalies are diagnosed.³⁷

Complete resolution of the pregnancy-related urinary tract alterations does not occur until 6 to 8 weeks' postpartum. Usually, pressure obstructive changes leading to hydronephrosis are alleviated immediately after delivery. The exception is those rare situations with posterior uterine fibroids that may continue to impinge on the ureters. Occasionally, a pelvic kidney may be present and should be identified.

Hepatic and biliary tract disease can complicate pregnancy any time during gestation. Gall bladder disease is the leading nonobstetrical reason for hospitalization during the first year of the postpartum period.³⁸ The hormonal changes predispose the pregnant woman to gallstone formation. Cholelithiasis is 4 times more common in women as compared to men. Glenn and McSherry reported that a majority of 300 women studied with gallstones had their initial presenting complaint during a pregnancy. Progesterone promotes gall bladder and biliary duct relaxation, leading to gall bladder enlargement; the enlarged gall bladder allows for stasis and stone formation.^{39,40, and 41} Most pregnant women who become symptomatic with cholelithiasis present in the third trimester. Presence of gallstones, sludge, gall bladder wall thickening, and biliary duct dilation are relevant sonographic findings.

The gall bladder wall can sometimes be edematous, particularly in severe preeclamptic women.⁴² The normal gall bladder wall measures less than 3 mm in thickness. A thickened gall bladder wall can be seen in patients with chronic cholelithiasis or cholecystitis and other causes unrelated to intrinsic gall bladder conditions. For instance, preeclampsia by virtue of its capillary leakage and hypoalbuminemia is associated with gall bladder mural thickening, which usually resolves within 1 week of delivery (Figure 30-10).

Clinically evident coagulopathies may occur with severe preeclampsia, HELLP syndrome (hemolysis, elevated liver enzymes, low platelets), and abruptio placentae. With severe preeclampsia and HELLP syndrome, liver edema and necrosis may occur. Sonography of the liver for subcapsular hematoma may be helpful in pregnant women with markedly elevated liver function tests or unexplained blood loss.^43,44 Care should be taken not to exert excessive pressure on the subcostal area to cause rupture of a subcapsular hepatic hematoma (Figure 30-11). For affected women with marked right upper quadrant tenderness, CT imaging is likely a superior choice.

Acute pancreatitis can complicate pregnancies due to gallstones or hyperlipidemia, and less commonly by alcohol use. The third trimester or postpartum period are times of particular predilection.⁴⁵ Presenting symptoms include nausea, emesis, midepigastric pain radiating to the back or lower abdomen, and fever. The serum amylase is frequently nondiagnostic; consequently, serum lipase is more specific. Sonographically, the inflamed pancreas is enlarged and shows decreased echogenicity. Pancreatic pseudocysts, phlegmons, and fluid collections are usually easily noted on sonography. Sonography is the imaging modality of choice in identifying gallstones and common bile duct enlargement.⁴⁶ These findings and other clinical clues may suggest an impacted stone at the ampulla of Vater for which endoscopic stone removal is frequently undertaken.

Pregnancy is accompanied by an increased risk of thromboembolic complications such as deep venous thrombosis and pulmonary emboli. In fact, thromboembolic disease is the most common cause of maternal mortality in the United States, occurring in about 1 per 1000 births.⁴⁷ Venous duplex Doppler studies of the lower extremities are useful for assessing lower extremity disease. Septic pelvic thrombophlebitis is a less common condition, presenting as fever, pelvic pain, leukocytosis, tachycardia, and sometimes a palpable pelvic mass or ovarian mass.^{48,49, and 50} This is usually a diagnosis of exclusion, and sonography may reveal an echogenic mass within a dilated vein. However, CT imaging with intravenous contrast or MR angiography is probably a better technique for diagnosis.⁵⁰

Clinical ultrasound has been used widely and has a definite role in the postpartum woman. The challenges include the rapid physiological changes, and the distortion of the anatomy by various pregnancy-related conditions. Appreciation of these changes by the clinician and sonologist allow for the appropriate diagnosis, differential diagnosis, and therapy.

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