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Monoamniotic twin pregnancy (including conjoined twins)

Charles J Lockwood, MD, MHCM
Anthony Odibo, MD, MSCE
Section Editors
Vincenzo Berghella, MD
Deborah Levine, MD
Deputy Editor
Vanessa A Barss, MD, FACOG


Monoamniotic twin pregnancies are the least common type of twin pregnancy. They have many of the same complications, but often at higher rates, as diamniotic twin pregnancies. In particular, they are characterized by a higher risk of fetal death.

This topic will discuss issues specific to monoamniotic twin pregnancies, including conjoined twins. General aspects of twin pregnancy are reviewed separately. (See "Twin pregnancy: Prenatal issues" and "Twin pregnancy: Labor and delivery".)


Monoamniotic twin gestations have a single placenta with one amnion and one chorion (figure 1). The two separate umbilical cords typically insert close to one another (within 6 cm of each other) and are located centrally in two-thirds of cases; the other one-third is either marginal or velamentous [1]. Intertwin placental vascular anastomoses are always present [2].


Timing of postfertilization division of the zygote determines placentation in twins. Monoamniotic, monochorionic placentation occurs with division at day 8 to 12; conjoined twins result from division at or after day 13. By comparison, diamniotic twins result from division earlier in gestation (day 4 to 8: diamniotic, monochorionic placentation; day 1 to 3: diamniotic, dichorionic placentation).

The factors responsible for timing of embryo division are not known. Use of assisted reproductive techniques appears to play a role as in vitro fertilization increases the frequency of monozygotic twinning. In some studies, manipulation of the zona pellucida, which is performed with intracytoplasmic sperm injection and assisted hatching, increased the frequency of monoamniotic twins [3-8]. Reports about atypical monoamniotic twins suggest the role of factors other than timing of fission. Mirror image twins are a subset of monoamniotic twins in which embryonic division occurs approximately on postfertilization day 9. These twins have mirror image features involving handedness and hair whorls, as examples; situs inversus of one twin has also been reported. Mirror image twinning has been attributed to division after the embryonic plate begins to lateralize [9,10].

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Literature review current through: Sep 2017. | This topic last updated: Aug 24, 2017.
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