The villi on the myometrial side of the conceptus are known as which one of the following?

Gestational Sac

Implantation usually occurs in the fundal region of the uterus between day 20 and day 23.21 In a study of early implantation sites in 21 patients, it was found that implantation occurs most frequently on the uterine wall ipsilateral to the ovulating ovary and least often on the contralateral wall.21 In addition, in a study of predominant sleeping positions in the peri-implantation period, Magann et al.22 found that the 33% of women who slept prone were most likely to have a high or fundal implantation than those who slept on their back or side. The latter groups predominantly had implantations corresponding to their resting posture.

At 23 days, the entire conceptus measures approximately 0.1 mm in diameter and cannot be imaged by TAS or TVS techniques. The earliest sonographic sign of an IUP was described by Yeh et al.,23 who identified a focal echogenic zone of decidual thickening at the site of implantation at about

The first reliable gray-scale evidence of an IUP is visualization of a small (1-2 mm fluid collection surrounded by an echogenic rim)gestational sac within the thickened decidua. Yeh et al.23 originally identified this sign, referred to as theintradecidual sign, which is seen at about 4.5 weeks' gestation. An intradecidual gestational sac should be eccentrically located within the endometrium. It is important to ensure that the sac abuts the endometrial canal to distinguish an intrauterine gestational sac from adecidual cyst.

The intradecidual sign was originally described on TAS,23 with a sensitivity of 92%, specificity 100%, and overall accuracy of 93% for distinguishing between early IUP and ectopic pregnancy. Chiang et al.24 looked at this sign using TVS and found overall sensitivity of 60% to 68%, specificity of 97% to 100%, and overall accuracy of 67% to 73%, indicating that the sign, when present, is useful for diagnosing an IUP. When absent, it does not reliably exclude an IUP. It is usually possible to demonstrate an early IUP as a small intradecidual sac between

Thedouble-decidual sign (also called double decidual sac sign) was described by Bradley et al.25 and Nyberg et al.26 as a method for distinguishing between an early IUP and an endometrial fluid collection of other origin, such as the pseudosac of an ectopic pregnancy. A well-defined double-decidual sign is an accurate predictor of the presence of an IUP. A vague or absent double-decidual sign should be considered nondiagnostic because it does not reliably exclude an IUP.27

Normal Early Intrauterine Pregnancy (IUP)

•

GS appears at ~ 5 weeks: Smooth, round or oval, fundal, ± intradecidual sac sign (IDSS) or double sac sign (DSS)

IDSS: Eccentric fluid collection in decidua, echogenic rim, curved edges

DSS: Concentric echogenic bands around most of GS

Signs absent in ≤ 35% of GS

Helpful if seen; however, absence does not exclude IUP

Normal IUP still possible if MSD 18-25 mm but no YS/embryo

Prominent color flow around sac: Low resistance, high velocity on spectral analysis of chorion

Variable and not necessarily useful, especially given theoretical risks to normal IUP with Doppler imaging

Gestational sac diameter

Transvaginal sonography enables visualization and evaluation of intrauterine pregnancies earlier than was previously thought possible. The earliest sonographic finding of an intrauterine pregnancy is thickening of the decidua. Sonographically, this appears as an echogenic, thick filling of the fundal region of the endometrial cavity occurring at approximately 3 to 4 weeks after the LNMP (Box 52-2,Figure 52-1).

At approximately 4 weeks of menstrual age, a small hypoechoic area appears in the fundus or midportion of the uterus, known as thedouble decidual sac sign. As the sac embeds further into the uterus, it is surrounded by an echogenic rim and is seen within the choriodecidual tissue. This is known as the chorionic orgestational sac.

At 5 weeks after the LMP, the average of the three perpendicular internal diameters of the gestational sac—calculated as the mean of the anteroposterior diameter, the transverse diameter, and the longitudinal diameter—can provide an adequate estimation of menstrual age (seeFigure 52-1). A gestational sac should be seen within the uterine cavity when the beta–human chorionic gonadotropin (beta-hCG) is above 500 mIU/ml (Second International Standard). This becomes especially important when evaluating a pregnancy for ectopic implantation.

The sac grows rapidly in the first 10 weeks, with an average increase of 1 mm per day. According to one report, a gestational sac growing less than 0.7 mm per day is associated with impending early pregnancy loss.16 Even the most experienced sonographer may incorporate a measuring error; therefore the beta-hCG test in conjunction with a sonographic evaluation is suggested in a sequential time frame.

When the gestational sac exceeds 8 mm in mean internal diameter, a yolk sac should be seen. The yolk sac is identified as a small, spherical structure with an anechoic center within the gestational sac. It provides early transfer of nutrients from the trophoblast to the embryo. It also aids in the early formation of the primitive gut and vitelline arteries and veins and in the production of the primordial germ cells. Yolk sac size has not been correlated with gestational age determination. Normal yolk sac size should be less than 5.5 mm. Yolk sacs greater than 5.6 mm have been associated with poor pregnancy outcome, as have solid, echogenic yolk sacs.Box 52-3 lists sonographic landmarks for early pregnancy.

When the meangestational sac diameter (GSD) exceeds 16 mm, an embryo with definite cardiac activity should be well visualized with transvaginal scanning. This usually occurs by the sixth menstrual week (Figures 52-2) but may be as early as the fifth LMP week with transvaginal sonography. For the transabdominal scanning approach, the maternal urinary bladder must be filled to create an acoustic window. With this technique, the sac shape can vary secondary to bladder compression, maternal bowel gas, or myomas and should not be misinterpreted as abnormal.

Normal gestational sac

The gestational sac is the implantation product that occurs in the uterus on approximately day 21. At that time the blastocyst is approximately 0.1 mm in size and cannot be seen by US. Normal sacs become visible when they reach 2 to 3 mm. Measurements:

Mean sac diameter (MSD)=(length+width+height)3

Normal MSD (mm) + 30 = days of pregnancy

After the gestational sac has developed, a yolk sac, the fetal heartbeat, and the embryo will become visible.

TVS Landmarks (Accuracy ± 0.5 Week)

Order of appearance of structures: gestational sac → yolk sac → embryo (fetal pole) → amnion (Fig. 10-7)

Gestational Sac, Yolk Sac, and Embryo

The gestational ages cited in the following discussion apply to TV sonography; the corresponding gestational ages using TA scanning would be higher. The gestational sac is the first structure identified in the uterus of an early pregnancy. A small gestational sac is generally visible at approximately 5 weeks gestation (Fig. 14-2A). The next structure visualized is the yolk sac, identified at approximately 5½ weeks. The yolk sac is depicted as a small, echogenic ring in the gestational sac (seeFig. 14-2B). The embryo is not visualized until after the yolk sac is at approximately 6 weeks gestation. The embryo is first identified as a focal thickening along the outer margin of the yolk sac (seeFig. 14-2C). Later in the first trimester, the yolk sac separates from the embryo and is found in the chorionic cavity, outside the amnion (seeFig. 14-2D). In the majority of normal pregnancies, embryonic cardiac activity is identified almost as soon as the embryo is visualized (Video 14-1

In the absence of an identifiable yolk sac or embryo, an intrauterine fluid collection should be assessed to determine whether it is an intrauterine gestational sac or a pseudogestational sac secondary to an ectopic pregnancy. An intrauterine gestational sac is likely when sonography reveals a small, rounded collection eccentrically located immediately adjacent to the uterine cavity, surrounded by a well-defined, thick, peripheral rim of echogenic tissue (Fig. 14-3A and B). This appearance has been termed theintradecidual sign.

Another pattern that favors an intrauterine gestational sac, termed thedouble decidual sac sign, is found slightly later in the first trimester than the intradecidual sign. The double decidual sac sign is composed of three discrete layers of echogenicity that surround much of the gestational sac. It is attributable to eccentric implantation of the gestation into the endometrium on one side of the uterine cavity. As the sac grows it deforms the uterine cavity, causing it to assume a curved configuration. An echogenic layer of decidual tissue develops on both sides of the uterine cavity and the uterine cavity is seen as a hypoechoic layer in between the curved echogenic lines (Fig. 14-4A and B). The inner echogenic layer (decidua capsularis) corresponds to the decidua immediately surrounding the gestational sac. The outer layer (decidua parietalis) corresponds to the decidualized endometrium on the opposite side of the uterine cavity from the gestational sac. The three-layer appearance is not seen at the site of implantation (decidua basalis).

Ultrasonographic Findings

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Gestational sac located high in fundus

Eccentrically located with respect to endometrial cavity

Sac seen separately > 1 cm from endometrial cavity

Appearance of sac contents quite variable

Gestational sac ± yolk sac, embryo

Gestational sac and embryo can be quite large

May appear as echogenic mass within cornua

Combination of trophoblastic tissue, hematoma

No definable sac

Thinned myometrium

< 5 mm of surrounding myometrium very suggestive

May have areas where no definable myometrium is seen

Normal myometrium may be seen early and does not exclude an interstitial ectopic

Early interstitial pregnancy often difficult to diagnose

42% of cases missed in 1 large series

Interstitial line sign has reported sensitivity of 80% and specificity of 98%

Echogenic line can be followed from endometrium to ectopic sac

More difficult to see as gestational sac enlarges

Best evaluated in transverse plane near fundus of uterus

Myometrial mantle sign

Myometrium surrounds sac in all planes

Becomes incomplete as sac enlarges

3D ultrasound shown to improve diagnosis

Improved spatial orientation of ectopic in relation to uterine cavity

Doppler findings

Trophoblastic tissue is highly vascular

Marked flow identified on color and power Doppler

Pulsed Doppler shows high-velocity, low-resistance waveform

May see prominent arcuate vessels in outer 1/3 of myometrium

Criteria for Diagnosing Very Early Intrauterine Pregnancy

The gestational sac can be detected sonographically when it is 2 to 3 mm in diameter, which corresponds to a gestational age between 29 days and 31 days.6–8 The appearance of a normal early gestational sac is a small fluid collection surrounded completely by an echogenic rim of tissue. As the sac enlarges, the echogenic rim, which is due to developing chorionic villi and adjacent decidual tissue, should be at least 2 mm thick, and its echogenicity should exceed that of the myometrium.9

To determine whether this small, saclike structure is an IUP and not a pseudosac associated with an EP, close inspection should be made of its position relative to the central cavity stripe of the endometrium. Because a true gestational sac implants into the thick decidualized endometrium, it should be located adjacent to the linear central cavity echo complex, and it should not displace or deform this echogenic anatomic landmark (Fig. 13-1). In 1986, Yeh and colleagues used this physiologic description of sac implantation to describe the intradecidual sign.10 In their investigation, which used a transabdominal approach, early IUP was diagnosed with a reported sensitivity of 92%, specificity of 100%, and accuracy of 93%.10 In a more recent effort to validate the effectiveness of this sign using a transvaginal approach, Laing and associates were not nearly as successful, with a reported sensitivity of 34% to 66%, specificity of 55% to 73%, and accuracy of 38% to 65%.11 On the basis of these relatively poor results, the authors recommend that in cases of very early pregnancy, follow-up sonography should be obtained to document conclusively the appearance of the yolk sac or embryo.

Gestational sac, yolk sac and fetal pole

The gestational sac may be recognized as early as 4 weeks and 1 day from the last menstrual period and should always be seen after 4 weeks and 4 days. Its diameter is about 2 mm and increases in size to measure 5–6 mm at 5 weeks.44 The mean gestational sac diameter then increases by approximately one millimetre per day throughout the first trimester.45 The yolk sac should be visible from 5 weeks’ gestation and increases in size to a maximum mean diameter of 5 mm at 10 weeks’ gestation. The majority of yolk sacs decrease in size before disappearing at around 12 weeks’ gestation. Some yolk sacs, however, will increase in size before disappearing.45 The fetal pole is usually visible towards the end of the 5th week and at 6 weeks the developing embryo appears as an echogenic line of about 5 mm tangentially touching and closely attached to the yolk sac (Fig. 4.1).26 The fetal heartbeat should be demonstrated from 6 weeks’ gestation (Fig. 4.2) and the heart rate is approximately 120 beats per minute. The amniotic sac is seen surrounding the developing fetus and the yolk sac lies outside the amniotic sac within the extra embryonic coelom (Fig. 4.3).

Gestational Sac and Yolk Sac Irregularities

An abnormally shaped gestational sac or yolk sac should raise concern for a failing pregnancy. The gestational sac should normally be round or oval. An irregular gestational sac with angulations or teardrop shape is associated with a poor prognosis (Figure 21-17). The decidual reaction around the sac may appear heterogeneous and cystic (see Figures 21-17 and 21-18, B).9 Likewise, yolk sac abnormalities, including enlargement (>6 mm), an irregular shape, and calcification (Figure 21-18), have been shown to correlate with early pregnancy failure.8 An abnormal position of the gestational sac in the lower uterine segment is another feature associated with poor outcome (Figure 21-19).6