Twin pregnancies: accuracy of first-trimester abdominal US in predicting chorionicity and amnionicity.

A first-trimester transabdominal ultrasound (US) study was performed on twin pregnancies to determine the utility of US in predicting chorionicity and amnionicity. Among 85 dichorionic-diamniotic (DC-DA) twin pairs, a thick membrane was present in 78 (92%). Four of the DC-DA cases without a thick membrane had two distinct placental sites, allowing 82 DC-DA pregnancies (96%) to be predicted. Among 16 monochorionic-diamniotic (MC-DA) twin pairs, a thin membrane was present in 14 (88%). None of the four monochorionic-monoamniotic (MC-MA) cases had an identifiable membrane. The lambda sign had no value in this evaluation and was actually misleading, while a thick membrane or the identification of two separate placentas was always predictive of DC-DA twinning. However, a thin membrane, while usually predictive of an MC-DA pregnancy, did not exclude a DC-DA gestation. When no membrane is present, an MC-MA gestation is probable; however, a diamniotic pregnancy may still be present, and further evaluation is suggested.