Chorionicity determination in twin pregnancies: how accurate are we?

Objectives To determine the accuracy of antenatal prediction of chorionicity in twin pregnancies in our institution. Methods Antenatal chorionicity was determined using the number of placental masses, the presence or absence of a twin peak sign and the fetal sex. The gestational age at assessment was documented. Postnatal diagnosis of chorionicity was determined by neonatal sex and placental Pathology. Results Chorionicity was correctly determined in 95% of cases (n = 131), 91% of the monochorionic and 96% of the dichorionic pregnancies were correctly determined. If chorionicity was assessed prior to 14 weeks' gestation (n = 96) the correct diagnosis was made in all except one case. Conclusions Ultrasound assessment of chorionicity has a high sensitivity and specificity. This is further improved if the assessment is performed prior to 14 weeks' gestation.