Gestational sac

Last revised by Jeremy Jones on 20 Sep 2021

The gestational sac (GS) 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 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.

See also

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Cases and figures

  • Case 1: gestational sac of 9w4d
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  • Case 2: small gestational sac
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