Introduction

Monoamniotic twins are a rare type of identical twins where the twins share the same amniotic fluid-filled sac.  The incidence of identical twins is about 1 in 250 pregnancies, and monoamniotic twins are about 1% of this number.  So monoamniotic twins are seen about 1 per 25,000 pregnancies.

Monoamniotic pregnancies are extremely high risk because the umbilical cords become entangled as the babies move around in their shared fluid sac.  These twins share one placenta, and the umbilical cords usually are attached to the placenta very close to each other.  According to one study the average distance between the cords is 1 1/2 inches.  For a picture of a monoamniotic placenta showing the closely attached and entangled umbilical cords, click here.

Umbilical cord "accidents" are the primary cause of fetal death.

All twin pregnancies are high risk, and these pregnancies should always be managed at a high risk clinic by a maternal-fetal medicine specialist.