replaced by the secondary yolk sac, which is the first anatomic struc­ture visualized within the gestational sac. The secondary yolk sac provides nutrients to the developing embryo and is the initial site of blood cell development. As illustrated below, the sonographic appear­ance of the yolk sac is small and round with bright, well-defined walls and an anechoic, fluid-filled center. Identification of the yolk sac is variable; however, it can be detected as early as 5 weeks GA with trans­vaginal transducers and should be visible by 7 weeks GA using a trans­abdominal approach. A faint flickering motion seen adjacent to the yolk sac represents the neurologically active heart tissue. From 5 to 10 weeks GA the yolk sac progressively increases to a maximum diam­eter of 5 to 6 mm. By the end of the first trimester, the yolk sac shrinks and is no longer appreciated sonographically.

cavity have enlarged enough to surround the embryo. The thin, reflective amniotic membrane encloses the developing echogenic embryo and “bathes” it in the anechoic amniotic fluid within the amniotic sac. As the trimester progresses, not only do the CRL and amniotic sac diameter increase 1 mm per day, but their measure­ments are equal. Between 12 weeks and 16 weeks GA, the chorionic cavity will be obliterated when the amniotic cavity has enlarged enough for the fusion of the amniotic and chorionic membranes. At this time, it may or may not be possible to distinguish an actual embryo crown (or cephalic pole) from a rump (or caudal pole) but the yolk sac should be seen lying outside the amniotic cavity. The yolk sac and embryo retreat from each other but remain connected by the yolk stalk or vitelline duct that eventually becomes part of the umbilical cord, the 3-vessel connection between the embryo/ fetus and mother. Also seen during this time is the tail-like append­age that is often identified at the site of the rump.

• As previously mentioned, the embryonic heart may be detected

placenta pre­sents with medium-sized indentations of the chorionic plate, a few small, linear, bright densities are identified at the basal layer, and the parenchyma contains bright, scattered, “commalike” densities. These findings are considered normal any time after 36 weeks of development. With grade III, the chorionic plate shows indenta­tions extending as far as the basal layer, dividing the placenta into segments. The basal layer exhibits very long, linear, bright, den­sities that may, in advanced stages appear as a bright, unbroken line. The placental parenchyma may contain highly echogenic and anechoic areas. The bright echoes represent large calcifications that may cast acoustic shadows. These findings are considered normal any time after 38 weeks of development.