Background Monochorionic pregnancies occur in 3.5/1000 pregnancies, but account for 50% of mortality in twin gestations. Assessment of chorionicity between 11 and 14 weeks is imperative to enable appropriate antenatal surveillance.

Objectives To determine the accuracy of chorionicity determination in twins in a multiple pregnancy clinic.

Methods A retrospective audit was conducted in the multiple pregnancy clinic at Leeds General Infirmary. Data was collected from 2004 to 2011.

Results Chorionicity was assessed in 512 sets of twins, but confirmed in 296: 203 on postnatal histology and 93 by discordant sex. Ultrasound examination correctly identified chorionicity in 289 (97.6%) twins.

Conclusions Ultrasound examination predicts chorionicity with a high degree of accuracy. The gold standard for confirmation of chorionicity is placental histology; however this was performed in only 60.4% of cases.