Scanning for chorionicity: comparison between sonographers and perinatologists

In most prenatal settings, twin pregnancies are initially evaluated by sonographers. Pregnancies diagnosed as monochorionic are subsequently referred to perinatologists or specialists in fetal medicine for the confirmation of chorionicity. In order to assess this screening strategy, we have compared the diagnosis of chorionicity made by the sonographers in the ultrasound department with the diagnosis done in the fetal medicine unit.

A cohort of women presenting with twin pregnancy and booked for prenatal care at University College London Hospitals over a 4-year period were investigated prospectively. All women were scanned at their initial visit at 11-14 weeks in the ultrasound department (US), and were subsequently referred to the Fetal Medicine Unit (FMU) for a second ultrasound evaluation. Ultrasound data were compared and diagnosis of chorionicity was confirmed by examination of the inter-twin membranes after delivery.

Chorionicity was determined in 172 twin cases by the two different departments. The overall rate of concordant chorionicity determination between both units was 90.1%. The rate of discordant results in dichorionic pregnancies was extremely small, 1 in 119 pregnancies (0.8%). The rate of discordant results for monochorionic diamniotic pregnancies was 5.5%. Monoamniotic pregnancies were over-diagnosed by the US technicians.

These results demonstrate that DC/DA chorionicity is accurately determined by sonographers at less than 14 weeks. In our opinion, it is both efficient and safe to rely on the diagnosis of the sonographers in DC/DA pregnancies in early pregnancy. In such pregnancies, a decision can be made either not to refer these patients for further evaluation of chorionicity by the fetal medicine team or to postpone the referral to after 14 weeks.