The extensive use of transvaginal sonography (TVS) has enabled improved evaluation of patients who are pregnant during the first trimester (up to 13 weeks) of pregnancy. TVS affords detailed delineation of the choriodecidua and embryo/fetus and distinguishes living from nonliving embryos/fetuses by establishing the presence of heart motion. It allows accurate assessment of the “developmental milestones” in first trimester pregnancy and reveals important information regarding abnormal or complicated early pregnancy. TVS can reveal detailed anatomic features of the fetus/embryo. Recently, 3D ultrasound has afforded even better anatomic resolution of the embryo and early fetus, both with volume and surface rendition (see Chapter 5).

This chapter discusses the role of TVS in the basic evaluation of an early first-trimester (before 13 weeks) pregnancy that is within the uterus. Chapter 4 is devoted to the sonographic evaluation of ectopic pregnancy. Chapter 5 describes sonographic assessment of the 11 to 18-week fetus, and Chapter 22 discusses the first-trimester screening for aneuploidy and associated anomalies. Recently, there have been major changes in the clinical and lab evaluations of first trimester with the use of cell-free DNA. Please refer to Chapter 5 for discussion of these changes. 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.¹ 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.¹ This condition is probably related to an extension of a retro- or subchorionic hemorrhage to involve more of the implantation site. The size of the retro- or subchorionic hemorrhage can be correlated to clinical outcome.²

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 (see Chapter 4). This can be accomplished best by transvaginal scanning that can document an intrauterine pregnancy (IUP) as early as 4 to 5 postmenstrual weeks.^3,4

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

1. 
   

   Establishment of intrauterine pregnancy, particularly when ectopic pregnancy is suspected

   
   
2. 
   

   Evaluation of complicated early pregnancy, such as subchorionic hemorrhage, incomplete abortion, early pregnancy failure, 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

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 high level of patient acceptance. Because of the theoretic potential for ascending infection, TVS should not be used when there is active bleeding and a dilated internal and/or 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 urinary bladder may assist in placing a very anteflexed uterus to a more neutral or horizontal position with TVS; however, greater degrees of bladder filling may actually displace the uterus away from the focal zone of the transducer, making detailed examination of the embryo and choriodecidua less optimal. Maximal bladder distension is not required for TVS. In fact, over distension of the bladder may degrade the resolution of TVS due to increasing the distance of the region of interest relative to the footprint of the transducer.

On a routine first-trimester TVS, certain structures should be clearly documented. These landmarks can be correlated to a general range of β-human chorionic gonadotropin (β-hCG) values.^5-7 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. 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.⁸

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 3-1A–D).

During the embryonic period, all 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.⁹ 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.¹⁰ This configuration is only present for 2 to 3 days.¹¹ 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.

Using data collected from patients undergoing in vitro fertilization, one study has indicated that a gestational sac can be seen routinely between 4 and 5 weeks of menstrual age.¹⁰ 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.¹² 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).^7,12 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.^3,13 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.¹¹ 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, 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 prominent relative to the rest of the body. A cystic area can be identified in the posterior aspect of the brain specifically the chorion fetus, representing the rhombencephalon.¹⁴ 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.