Radiology, Vol 185, 759-762, Copyright © 1992 by Radiological Society of North America
Twin pregnancies: accuracy of first-trimester abdominal US in predicting chorionicity and amnionicity
AB Kurtz, RJ Wapner, J Mata, A Johnson and P Morgan
Department of Radiology, Jefferson Medical College, Philadelphia, PA 19107-5091.
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.
