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Monoamniotic twin pregnancy

Henry Roque, MD, MS
Charles J Lockwood, MD, MHCM
Section Editor
Susan M Ramin, MD
Deputy Editor
Vanessa A Barss, MD, FACOG


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 'Prevalence 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.


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Literature review current through: Oct 2015. | This topic last updated: Jun 2, 2015.
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