Monoamniotic twin pregnancies result from division of a single fertilized oocyte. The fetal membranes consist of one amnion and one chorion (figure 1). These pregnancies are relatively rare and characterized by a high risk of fetal complications, which will be reviewed in the following discussion.
The incidence of monoamniotic twins is approximately 1 in 10,000 pregnancies. Monoamnionicity is the least common pattern of twin placentation, affecting only 1 to 5 percent of monozygotic twin gestations [1-3]. (See "Twin pregnancy: Prenatal issues", section on 'Incidence and epidemiology'.)
Timing of postfertilization division determines placentation in twins:
●Diamniotic, dichorionic placentation occurs with division prior to the morula stage (within three days post fertilization).
●Diamniotic, monochorionic placentation occurs with division between days 4 and 8 post fertilization.