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The gestational sac is the first sign of early pregnancy on ultrasound and can be seen with endovaginal ultrasound at approximately 3-5 weeks gestation when the mean sac diameter (MSD) would approximately measure 2-3 mm in diameter.
A true gestational sac can be distinguished from a pseudogestational sac by noting:
its normal eccentric location: it is embedded in the endometrium, rather than centrally within the uterine cavity
presence of the double decidual sign (most helpful at 4.0-6.5 weeks)
presence of a yolk sac: seen at approximately 5.5 weeks (unequivocal evidence of a gestational sac)
However, caution should be exercised with a diagnosis of a pseudogestational sac. In a woman with a positive beta-hCG, any intrauterine sac-like fluid collection seen on ultrasound is highly likely to be a gestational sac 3.
Some suggest that if the beta-hCG level is >1800 mIU/mL (second international standard) a transabdominal ultrasound should be able to identify an intrauterine gestational sac; however, this value is debatable, and beta-hCG levels >2000 (or even >3000) without a visualized intrauterine gestational sac does not rule out an intrauterine pregnancy 4. Treatment for a suspected failed pregnancy should not be initiated on a hemodynamically stable woman on the basis of a single beta-hCG level 5.
If one cannot identify a yolk sac at a mean gestational sac diameter of 16-24 mm, this is suspicious for, though not diagnostic of, a failed early pregnancy.
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