TABLE 2953 Earliest Sonographic Visualization of Embryonic Structures

The primary ultrasound finding to be evaluated in first-trimester patients is evidence of an intrauterine pregnancy. The earliest sonographic finding of pregnancy is the gestational sac. This appears as a round or oval anechoic area within the uterus ( Fig.i295:17). Care should be exercised in identifying gestational sacs, since up to 20 percent of ectopic pregnancies have a "pseudogestational sac," which looks sonographically similar to a true gestational sac but is caused by intracavitary blood surrounded by a decidual reaction. True gestational sacs have two concentric echogenic rings surrounding the gestational sac ( double decidual sign), representing the decidua parietalis and the decidua capsularis, and tend to appear to be adjacent to, not within, the echogenic endometrial complex ( intradecidual sign). The first true embryonic structure that can be seen sonographically is the yolk sac ( Fig, 295:9). This echogenic ringlike structure can be seen with endovaginal scanning at approximately 5 weeks after the LMP. As the pregnancy progresses, the fetus is seen at 5.5 to 6 weeks, with fetal parts being visible at 8 weeks endovaginally. Fetal cardiac activity can generally be seen from the time the fetal pole is identified but should always be present when the embryo is greater than 10 mm if the fetus is viable. The presence of fetal cardiac activity is very helpful prognostically. The rate of spontaneous abortion is less than 5 percent when normal cardiac activity is seen at 8 to 12 weeks of gestation.

FIG. 295-17. Normal intrauterine pregnancy. The fetal pole (within calipers) and the round, echogenic yolk sac are seen within the anechoic gestational sac.

Although the primary goal of bedside US is to identify an intrauterine pregnancy, ectopic pregnancies can result in a number of sonographic findings. First, a living extrauterine pregnancy may be seen, obviously securing the diagnosis of ectopic pregnancy. This occurs in approximately 20 percent of ectopic pregnancies. The sonogram may also demonstrate abnormal structures suggestive of an ectopic pregnancy, such as an echogenic ringlike mass in the adnexae. The most common finding seen with ectopic pregnancies is that of an empty uterus with either no other findings or nonspecific findings (e.g., fluid in the cul-de-sac).

The quantitative b human chorionic gonadotropin (b-HCG) level is extremely helpful in evaluating pregnant patients with an empty uterus on ultrasound. A number of authors have suggested "discriminatory zones," b-HCG levels above which an intrauterine pregnancy should be visualized if present. The value utilized in clinical practice depends on the skill of the sonographer, the quality of the ultrasound machine, and individual patient factors. Based on the work of several authors, it is expected that intrauterine pregnancy is detectable on endovaginal scanning if the b-HCG level is greater than 2000 MIU/mL. 11 In some institutions, the discriminatory zone could be as low as 1000 MIU/mL. Patients with b-HCG levels greater than this (the discriminatory zone) who do not have evidence of an intrauterine pregnancy on ultrasound are at very high risk for an ectopic pregnancy, and obstetrical consultation is needed. Such patients may undergo observation with repeat b-HCG level determinations, and ultrasound, laparoscopy, or surgical intervention, depending on the individual patient and the preferences of the obstetrician. For hemodynamically stable patient's with a b-HCG level less than 2000 MIU/mL and no intrauterine pregnancy on ultrasound, disposition is usually arranged in concert with the obstetrician. If the patient is discharged, obstetric follow-up is needed in 48 to 72 h, whereupon repeat b-HCG and ultrasound examinations are performed. Mateer and colleagues followed such a protocol and were able to decrease the number of delayed diagnoses of ectopic pregnancy from 43 to 28 percent, and the incidence of ectopic rupture at diagnosis decreased from 50 to 9 percent.11

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