Threshold, Discriminatory Zone, and “The New Rules”

Abstract

The sonographic evaluation of early pregnancy has dramatically improved since the introduction of transvaginal sonography. Relatively recent publications highlight the importance of caution in evaluating and intervening in early pregnancy. As a result, new recommendations have been established for identifying milestones in early singleton pregnancy, including a threshold level of hCG = 390–1000 mIU/mL for visualizing a gestational sac, and a discriminatory value of 3000 IU/mL for identifying all normal intrauterine pregnancies. These same values do not apply to multiple gestations, and thus, caution is advised in patients who have undergone assisted reproduction.

The ultrasound findings consistent with early pregnancy failure have also changed. To assure that one does not intervene on a potentially viable early intrauterine pregnancy the following guidelines are recommended to definitively establish early pregnancy failure: (1) the lack of a fetal heartbeat with a crown-rump length ≥7 mm; (2) a mean sac diameter of ≥25 mm with no embryo; (3) absence of an embryo with a heartbeat ≥2 weeks after an ultrasound revealed a gestational sac without a yolk sac; and (4) absence of an embryo with a heartbeat ≥11 days after an ultrasound revealed a gestational sac with a yolk sac. These recommendations incorporate the interobserver and intraobserver variation of ultrasound measurements found in multiple studies.