Prenatal diagnosis of holoprosencephaly by transvaginal ultrasonography in the first trimester.

A diamniotic-monochorionic pregnancy of twins was revealed by transvaginal ultrasonography: one fetus with holoprosencephaly was diagnosed at 11 weeks of gestation. This is the first case of holoprosencephaly diagnosed in the first trimester. Prenatal diagnosis of this anomaly in the first trimester is now possible by transvaginal ultrasonography.