Chapter 3: TRANSVAGINAL SONOGRAPHY OF EARLY INTRAUTERINE PREGNANCY

Transvaginal sonography (TVS) has become the method of choice for the detection and evaluation of early pregnancy. It affords detailed delineation of the choriodecidua and embryo/fetus and distinguishes living from nonliving embryos/fetuses by establishing the presence of heart motion. Most of the sonographic milestones used for assessment of early pregnancy can be established 1 week earlier with TVS than with conventional transabdominal sonography (TAS).¹

This chapter discusses the role of TVS in the basic evaluation of a first-trimester pregnancy that is within the uterus. Chapter 4 is devoted to the sonographic evaluation of ectopic pregnancy. Chapter 22 describes first-trimester screening for aneuploidy and anomalies. The American Institute of Ultrasound in Medicine (AIUM) Guidelines for Obstetrical Sonography in the First 10 Weeks is included as Appendix 3-1.

Transvaginal sonography has several clinical indications in the first trimester of pregnancy. The majority of these involve the establishment of the location of the pregnancy and the detection of embryonic/fetal life. Other indications include establishing the cause of vaginal bleeding and the prognosis of the pregnancy.

Approximately 20% to 50% of patients may experience bleeding in the first few weeks of pregnancy.² This bleeding has been attributed to the anchoring of the choriodecidua as the blastocyst burrows into the decidualized endometrium. This bleeding is usually limited and not associated with uterine cramping. On the other hand, 20% to 30% of patients with bleeding will progress to a threatened abortion.² This condition is probably related to an extension of a retrochorionic hemorrhage to involve more of the implantation site. The size of the retrochorionic hemorrhage can be correlated to clinical outcome.³

Transvaginal sonography has a major role in evaluating patients with suspected ectopic pregnancy. Most importantly, TVS can accurately establish that the pregnancy is intrauterine, virtually excluding the possibility it is ectopic. This can be accomplished best by transvaginal scanning that can document an intrauterine pregnancy (IUP) as early as 4 to 5 postmenstrual weeks.^4,5

Thus, the major indications for TVS in the first trimester include

1. Establishment of intrauterine pregnancy, particularly when ectopic pregnancy is suspected

2. Evaluation of complicated early pregnancy, such as retrochorionic hemorrhage, incomplete abortion, early pregnancy failure with resorption, or completed abortion

3. Detection of embryonic/fetal life

4. Precise localization of intrauterine contraceptive devices (IUCDs) associated with early pregnancy

5. Detection of multiple gestations when clinically suspected

In most cases, TVS is the method of choice over TAS for evaluation of first-trimester pregnancies. This is primarily because of its improved resolution of the intrauterine contents and increased patient acceptance.⁴ Because of the theoretic potential for ascending infection, TVS should not be used when there is active bleeding and a dilated external cervical os. Transabdominal sonography still is an accurate means for confirmation of location and confirmation of a viable pregnancy greater than 8 to 10 weeks, and it can be used solely or in conjunction with TVS.

The technique for TVS begins with placing some ultrasonic coupling gel within a sheath such as a condom and covering the disinfected transducer with the sheath. The transducer is lubricated and then inserted through the introitus and into the midvagina. When placed within the vagina, the transducer can be manipulated in the semicoronal and sagittal planes for delineation of the uterus and in the adnexa in long and short axes. A slightly distended bladder may assist in placing a very anteflexed uterus to a more neutral or horizontal position; however, greater degrees of bladder filling may displace the uterus away from the focal zone of the transducer, making detailed examination of the embryo and choriodecidua difficult.

On a routine first-trimester TVS, certain structures should be clearly documented. These landmarks can be correlated to a specific range of β-human chorionic gonadotropin (β-hCG) values.^{6,7, and 8} Included in the evaluation are the position and regularity of the choriodecidua of the gestational sac; the presence or absence of a yolk sac, embryo, or both; and the evaluation of the adnexa and cul-de-sac. When an embryo is identified, its crown-rump length (CRL) should be measured accurately. If an embryo cannot be delineated, gestational sac dimensions are useful alternative parameters for measurement to determine gestational duration.⁹ For this measurement, the 3 inner-to-inner dimensions (long, short, and anterior-posterior) are obtained and then averaged. Prior to depicting an embryo, the sonographic documentation of a yolk sac within the gestational sac is a reliable means to confirm that the pregnancy is indeed intrauterine.¹⁰

Although TVS is usually sufficient in early pregnancy, occasionally structures that are superior to the uterus and outside the field of view of the transducer may be difficult to image. For these, a routine transabdominal scan with a fully distended bladder may be helpful.

Normal First-Trimester Pregnancy

This discussion of normal development is divided into discussions of 4 to 6 weeks, 7 to 8 weeks, and 9 to 11 weeks (Table 3-1 A,B,C, and D).

During the embryonic period, all of the main viscera are formed. In the fetal period, these formed structures grow and complete their functional development. This distinction is somewhat arbitrary and is based on terminology used in embryology. The terms used by embryologic texts, specifically gestational age, differ in meaning from those used clinically. Embryologic texts typically describe development in terms of the time from conception (gestational age), whereas menstrual age is used in a clinical setting because it dates from a recordable event. Although there is usually a 2-week interval between the time of fertilization and the last day of menses, this can vary by ±8 days. The events described in this chapter are classified by their menstrual dates.

4 to 6 Weeks

The midembryonic period of development can generally be defined from the fourth to sixth menstrual weeks (Figures 3-1 and 3-2). The embryonic anatomy present in early embryonic development is generally below the resolution of most currently available systems. Variations in the time of ovulation (up to 12 days) and implantation (up to 3 days) may influence what is depicted on a transvaginal scan in this early stage of pregnancy.

Using TVS, one of the first signs of an IUP is a hypoechoic complex within the thickened decidualized endometrium. This complex measures only a few millimeters. The gestational sac can be identified as early as 4 weeks and 3 days, but should be routinely detected by TVS after 5 weeks.¹¹ Within the sac, a double sac structure measuring a few millimeters, which represents the developing primary yolk sac and extraembryonic coelom (double bleb), can be seen surrounded by the echogenic layer of choriodecidua at 5 weeks.¹² This configuration is only present for 2 to 3 days. The embryo, which is not visible at this stage, is termed a trilaminar embryo because, microscopically, 3 distinct layers (endoderm, mesoderm, and ectoderm) are present.

Because TVS is a relatively new clinical modality, additional experience with it is needed before absolute standards for sac size relative to yolk sac/embryo visualization are established. Using data collected from patients undergoing in vitro fertilization, 1 study has indicated that a gestational sac can be seen routinely between 4 and 5 weeks of menstrual age.¹² Our experience indicates that a gestational sac can be identified as early as 4 weeks and 2 days menstrual age. In general, a yolk sac can usually be demonstrated within the gestational sac by TVS when the sac is approximately 1 cm in size; an embryo/yolk sac is usually seen in sacs that average 1.5 cm.¹⁴ Similarly, preliminary experience has suggested that the β-hCG level at which early gestational sacs are seen by TVS is in the range of 500 to 800 mIU (the second international standard). The most recent international standard is 1500 to 2400 mIU. This is significantly lower than the level reported with TAS (1800 to 3000 mIU).^8,14 The gestational sac itself grows approximately 1 to 2 mm in size each day at this time and can usually be delineated within the thickened decidua vera. Changes in the gestational sac can be seen within 3 to 5 days of the initial screen.

During the middle of the fifth postmenstrual week (31/2 weeks of gestational age), the embryo measures between 2 and 5 mm and is located adjacent to the relatively prominent secondary yolk sac, which appears as a rounded hypoechoic structure between 3 and 4 mm in size. An enlarged yolk sac (>6 mm) is associated with embryonic demise as well as those that are compromised and small.¹⁵ The embryo/yolk sac complex lies adjacent to the edge of the gestational sac and has been described as forming a "double bleb," representing the amniotic sac-embryo/yolk sac complex.¹³ By the end of the first half of the embryonic period, the choriodecidua forms the boundaries of the gestational sac, which appears as an echogenic ring of tissue. At 4 weeks of menstrual age, the gestational sac measures only 3 to 5 mm in diameter and grows to approximately 1 cm at 5 weeks.

During the early embryonic period the embryo may be barely visible on TVS. Although many of the structures are present, they cannot be resolved sonographically. The neural tube is closed in its midportion but open at its rostral and caudal ends. Brachial arches form, and the somites develop as rounded surface elevations. Forty-two or forty-four somites form; these paired structures eventually give rise to the axial skeleton and associated musculature.

7 to 8 Weeks

During the latter half of the embryonic period, sonographic scanning can depict a gestational sac, the developing embryo and its heartbeat, the surrounding membranes, and the choriodecidua. During this period, organogenesis of the major body viscera occurs (Figures 3-3,3-4,3-5, and 3-6).

On both TVS and TAS, heart pulsations can be depicted during this period of gestation. Transvaginal sonography is most precise in depicting early heart pulsation after 6 postmenstrual weeks, when the developing embryo forms from 2 enfolding fusiform tubes and begins contractile activity.

During the seventh postmenstrual week (fifth week of gestational age), the developing embryo grows from 6 to 11 mm in CRL. During this phase of development, the head growth is extensive and results primarily from rapid development of the brain. A cystic area can be identified in the brain, representing the rhombencephalon.¹⁶ The yolk sac is relatively large, measuring less than 6 mm inner-to-inner dimensions, and floats within the gestational sac between the chorion and amnion, attached to the developing umbilical cord.

During the eighth postmenstrual week of embryonic development (6 weeks of gestational age), the embryo grows from 14 to 21 mm in length. The head remains a large and prominent structure and is bent over the heart prominence. The yolk sac becomes progressively smaller, and the intestines enter the base of the umbilical cord, beginning the normal process of umbilical herniation. By the end of the ninth postmenstrual week (seventh week of gestational age), the embryo has attained human features.¹⁷ The head, body, and extremities can be identified sonographically. The intestine is still within the proximal portion of the umbilical cord. Occasionally, this physiologic umbilical herniation of bowel is particularly well depicted with TVS. Because this process of physiologic herniation of bowel into the umbilical cord is normal, abnormalities of the ventral wall should be suspected only if the bowel remains outside of the abdomen at 12 weeks or beyond.

Another structure that can be depicted in the late embryonic period is the amniotic membrane.¹⁸ The amniotic cavity forms from an area deep in the trilaminar embryo, and the amniotic membrane can be seen on a fully floating linear interface in the outer portion of the amniotic cavity. The amnion approximates with the chorion only late in the first trimester of pregnancy (14 to 18 weeks).¹⁹ At 6 to 8 weeks, the membrane can be seen as a thin, rounded structure that encircles the embryo/fetus on TVS. Prior to this, the amniotic membrane may appear as a linear echogenic interface projected within the gestational sac in proximity to the embryo.

Besides depiction of the embryo/fetus, the choriodecidua is seen as it begins to thicken at the implantation site during the late embryonic and early fetal period. The anatomic and functional fusing of decidua basalis and chorion frondosum forms the future placenta.

Certain parameters provide useful prognostic signs, including the heart rate and the relative size of the embryo to the amniotic sac.

9 to 11 Weeks

After 9 weeks, the fetus is clearly depicted both with TAS and TVS. Nomograms for measurement of the embryo and fetus have been established.²⁰ The fetus begins to move its trunk and extremities, and it can be seen to do an occasional somersault within the uterus. Movement is rapid in nature and often appears convulsive. Upper extremity movement is followed by lower extremity. The fetal brain has relatively large lateral ventricles that are mostly filled with choroid plexus (Figures 3-5,3-6,3-7,3-8,3-9, and 3-10). Small cysts within the umbilical cord can be seen but usually are resolved by 12 weeks.²¹ Herniated bowel also returns into the abdomen by 12 weeks. Before 12 weeks, however, the physiologically herniated bowel can measure up to 1.5 times the umbilical cord at its abdominal insertion. Color Doppler sonography may be used to assess the size of the herniated bowel in relation to the cord.

Heart rate progressively increases to 120 to 160 beats per minute after 6 to 7 weeks.²¹ Heart rates of less than 85 beats per minute have been associated with pregnancy failure and necessitate follow-up sonograms.²² In another study, heart rates of less than 90 beats per minute in the first trimester were associated with a dismal diagnosis.²³ Clearly, however, one could give the fetus the benefit of the doubt if slow heart rates are seen and confirm this finding on a follow-up study rather than terminate based on one abnormal examination.²⁴ Another parameter that seems to have prognostic value is the size of the amniotic sac relative to the embryonic length. The yolk sac is typically 6 mm or less in normal pregnancies.²⁵ An enlarged amniotic sac may be seen with embryonic demise as calculated by CRL − D_a > 0.8 cm (diameter of the amniotic cavity).²⁶ In normal pregnancies, the amniotic sac minus embryonic length should be greater than 5 mm.²³ This measurement is less helpful because it may be difficult to completely visualize the amnion at this early stage of development.

Several studies have shown a gradual increase in velocity and diastolic flow in choriodecidual (spiral) arteries in early pregnancies.^27,28 However, the actual Doppler indices do not discriminate between viable and nonviable pregnancies. Increased venous flow within the choriodecidua can be seen in nonviable pregnancies associated with embryonic or early fetal demise.

Failed or failing early pregnancies seem to be more vascular than normal gestation.²⁹ Thus, color Doppler sonography (CDS) may help define the etiologic mechanism for early pregnancy failure. However, uncomplicated involution of the uterus may contain echogenic material with low-impedance flow.³⁰ Thus, the role of CDS is yet to be detemined.

Another study has reported that pregnancy failure is more common when the retrochorionic hemorrhagic is over two-thirds the size of the gestational sac, when the patient is over 35 years old, and when the pregnancy is less than 8 weeks.

As stated previously, it is not unusual for the pregnant patient to experience painless spotting in the first few weeks of pregnancy. This probably is related to trophoblastic implantation within the decidualized endometrium. As the gestational sac develops, small (2- to 5-mm) hypo-echoic areas may be seen immediately beneath the echogenic choriodecidua that probably represent areas of blood pools or lacunae (see Figure 3-8).

On transvaginal color Doppler sonography (TV-CDS), arterial and venous flow can be seen within the choriodecidua prior to sonographic visualization of the embryo (Figure 3-11). Arterial velocities within the choriodecidua gradually increase.

Failing or failed IUPs tend to demonstrate an increase in venous flow beneath the choriodecidua (see Figure 3-11). There does not seem to be a statistically significant difference in their arterial velocities in normal versus abnormal early pregnancies, however.

Patients who present with extensive bleeding may have retrochorionic hemorrhage. In this disorder, there is more extensive bleeding behind the chorion, which appears as a hypoechoic area surrounding the gestational sac. Using the formula for a prolate ellipse volume (cc)—length (cm) × width (cm) × height (cm) × 0.5—the relative size of the retrochorionic hemorrhage can be quantified in relation to the size of the gestational sac itself. It has been shown that the relative size of the retrochorionic hemorrhage has some implications as to whether or not the pregnancy will progress.³¹ When the area of the retrochorionic hemorrhage is less than one-fourth of the gestational sac or less than 60 mL, it is likely that the pregnancy will progress.³

In spontaneous incomplete abortions, there is usually passage of the fetus or embryo with retained choriodecidua. This tissue typically appears as echogenic material within the uterine lumen. The choriodecidua is irregular, and the gestational sac itself appears "deflated" or irregular in shape.

In cases of failed embryonic development, there is a failed or abnormal development of the embryo and its associated umbilical cord and body stalk. Thus, even though a gestational sac may appear normally formed, no embryo or, on occasion, no yolk sac will be identified within the uterus. An early pregnancy failure is usually a reflection of a chromosomally aberrant conceptus.

Embryonic demise can be documented by TVS when there is lack of heart motion in an embryo that measures over 6 mm in length. In general, heart motion can usually be detected if an embryo can be delineated.³² In some cases of failed embryonic development, amorphous internal debris can be present within the sac. They probably represent strands of blood or sloughed decidual tissues.

In completed miscarriages, there is close apposition of relatively thin and regular endometrial interfaces. Although one might argue that ectopic pregnancies may demonstrate this appearance, correlation with β-hCG values may be helpful in confirming a completed miscarriage. In completed miscarriage, serial β-hCG values will typically fall precipitously, whereas in ectopic pregnancy this value slowly decreases or reaches a plateau.³³ In induced abortions, β-hCG was detectable from 16 to 60 days with a mean of 30 days after uterine evaluation, as opposed to spontaneous abortions where it was detectable from 9 to 35 days with a mean of 19 days, and in ectopic pregnancies, it was detectable for 1 to 31 days with a mean of 8 days, 5 days after laparoscopic tubal removal.³²

Nature has a healthy range of normal variations. So it is true for sonographic features of early pregnancy.

One variant that can be confused for a pathologic condition involves overgrowth of chorion to simulate the sonographic appearance of a polyp or abnormal embryo. This condition has been termed a chorionic "bump."³⁴ Figure 3-12 illustrates a patient whose transvaginal sonogram was initially interpreted as representing a demised embryo. However, on repeat TVS, the structure thought to represent the demised embryo was found separate from a normal yolk sac/embryo.

It is important in some patients with IUCDs and complicated pregnancies to confirm the presence of an IUCD and establish its location relative to the gestainal sac in the developing choriodecidua. Clearly, IUCDs that are implanted superior to the gestational sac are more difficult to extract than those that are inferior. The amount of retrochorionic hemorrhage associated with an IUCD can also be quantified using TVS.

Transvaginal sonography may also be helpful in the evaluation of first-trimester pregnancies complicated by trophoblastic disease. Although large hydropic villi may not be present in trophoblastic disease at this stage, the abnormal tissue can be diagnosed as well as its relative amount. Trophoblastic tissue, however, frequently has the sonographic appearance of retained choriodecidua. This disorder is discussed in detail in Chapter 29.

The recent development of TVS has been discussed in this chapter. Transvaginal sonography primarily has a role in the diagnosis of early intrauterine pregnancy in patients suspected of ectopic pregnancy and in detecting embryonic or fetal life in those patients with extensive bleeding, cramping, or both in the first trimester. Transvaginal color Doppler sonography can be useful as a problem solver in selected cases (Figs. 3-13,3-14,3-15,3-16, and 3-17). 3D transvaginal sonography may also be helpful in certain cases (Figure 3-19).

INTRODUCTION

This portion of the guideline has been developed for use by practitioners performing sonographic studies only during the first 10 menstrual weeks of pregnancy. Such sonography should be performed only when there is a valid medical reason, and the lowest possible ultrasonic exposure settings should be used to gain the necessary diagnostic information. A limited examination may be performed in clinical emergencies or in specific clinical scenarios, such as evaluation of fetal or embryonic cardiac activity. A limited follow-up examination may be appropriate if a complete prior examination is on record. While this guideline describes the key elements of standard sonographic examinations in the first 10 weeks of pregnancy, in some cases, other specialized examinations may be necessary as well.

SPECIFICATIONS OF THE EXAMINATION

1. Indications

A sonographic examination can be of benefit in many circumstances in the embryonic period of pregnancy, including but not limited to the following indications:

1. To confirm the presence of an intrauterine pregnancy

2. To evaluate a suspected ectopic pregnancy

3. To define the cause of vaginal bleeding

4. To evaluate pelvic pain

5. To date the pregnancy

6. To diagnose or evaluate multiple gestations

7. To confirm cardiac activity

8. To evaluate maternal pelvic masses and/or uterine abnormalities

9. To evaluate a suspected hydatidiform mole

Comment

A limited examination may be performed to assess the presence of cardiac activity.

2. Imaging Parameters

Overall Comment

Scanning in the first 10 weeks of pregnancy may be performed either transabdominally or transvaginally, although transvaginal scanning is preferred. Patients should be questioned about latex allergy prior to use of a latex sheath.

• a. The uterus, including the cervix, and adnexa should be evaluated for the presence of a gestational sac. If a gestational sac is seen, its location should be documented. The gestational sac should be evaluated for the presence or absence of a yolk sac or embryo, and the embryonic size should be measured and recorded, when possible.

Comment

Embryonic size is a more accurate indicator of gestational (menstrual) age than mean gestational sac diameter. However, the mean gestational sac diameter may be measured and recorded when an embryo is not identified. Caution should be used in making the presumptive diagnosis of a gestational sac in the absence of a definite embryo or yolk sac. Without these findings, an intrauterine fluid collection could represent a pseudo-gestational sac associated with an ectopic pregnancy.

• b. Presence or absence of cardiac activity should be reported.

Comment

With transvaginal scans, cardiac motion is usually observed when the embryo is 5 mm or greater in length. If an embryo less than 5 mm in length is seen without cardiac activity, a subsequent scan at a later time may be needed to document cardiac activity. If possible, the M-mode function of the scanner should be used to document cardiac activity.

• c. Embroyonic number should be reported.

Comment

Amnionicity and chorionicity should be documented for all multiple pregnancies when possible.

• d. Evaluation of the uterus, adnexal structures, and cul-de-sac should be performed.

Comment

The presence, location, and size of adnexal masses should be recorded. The presence of leiomyomata should be recorded, and measurements of the largest or any potentially clinically significant leiomyomata should be recorded. The cul-de-sac should be scanned for the presence of absence of fluid.

3. Equipment Specifications

These studies should be conducted with real-time scanners, using a transabdominal and/or a transvaginal approach. A transducer of appropriate frequency should be used.

Comment

Real-time sonography is necessary to confirm the presence of cardiac activity. A transvaginal scanning approach is preferred for this indication.

4. Fetal Safety

Diagnostic ultrasound studies of the fetus are generally considered to be safe during pregnancy. This diagnostic procedure should be performed only when there is a valid medical indication, and the lowest possible ultrasonic exposure setting should be used to gain the necessary diagnostic information under the as low as reasonably achievable (ALARA) principle.

The promotion, selling, or leasing of ultrasound equipment for making "keepsake fetal videos" is considered by the US Food and Drug Administration (FDA) to be an unapproved use of a medical device. Use of a diagnostic ultrasound system for these purposes, without a physician's order, may be in violation of state laws or regulations.