The blastocyst begins to implant in the endometrium at 3 weeks menstrual age (1 week postconception),1 the first definitive ultrasound sign of pregnancy is the “gestational sac”. Prior to the appearance of the gestational sac, the endometrium is markedly echogenic and the arcuate vessels are somewhat prominent.2 This, however, is nondiagnostic and can often be seen in the normal late secretory phase. In ultrasound images the gestational sac appears as a thick echogenic ring surrounding a sonolucent centre (Figure 34. 1). This sonolucent centre is actually the fluid-filled chorionic sac.2 This sac already contains the amnion, bilaminar embryonic disc and yolk sac, but these structures are too small to be imaged, even with the high magnification of our current endovaginal probes. The echogenic ring is the result of a trophoblastic decidual reaction. Early on the entire chorionic sac is surrounded by chorionic villi (Figure 34. 2). These villi are symmetrically located. Some villi will bud and branch into secondary and tertiary villi, and become chorion frondosum (the forerunner of the placenta). Other primary villi will regress and become chorion leave.


Ectopic Pregnancy Chorionic Villus Intrauterine Pregnancy Spontaneous Passage Pregnancy Failure 
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© Springer-Verlag London Limited 2003

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  • Steven R. Goldstein

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