Monoamniotic twins diagnosed by ultrasound in the first trimester

A case of monoamniotic twins was diagnosed by ultrasonic scanning in the first trimester through the presence of two fetuses in one gestation sac. The fetuses died in utero in weeks 17 and 25. The possible ultrasound appearance of the various types of monozygotic and of dizygotic twins in early pregnancy is discussed and a scheme is suggested which allows some differentiation.