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Key Points

  • Monoamniotic twinning is a rare form of twinning where both twins occupy a single amniotic sac.

  • This form of twinning occurs when a single embryo splits during postovulation days 8 to 10.

  • The most common way to diagnose monoamniotic twins antenatally is failure to visualize a dividing membrane in the presence of a single placental mass and like-gender twins. During the first trimester, the presence of two embryos in a single sac, but with only one yolk sac, is highly suggestive of monoamnionicity.

  • These pregnancies are at risk for significant perinatal morbidity and mortality. This has been attributed to preterm delivery, intrauterine growth restriction, congenital anomalies, vascular anastomoses between twins, and umbilical cord entanglement or cord accidents.

  • Intense antenatal surveillance and timed delivery has been shown to improve outcomes. This typically takes the form of elective hospitalization with daily nonstress testing at 24 to 26 weeks’ gestation, and increased frequency of testing if variable decelerations are noted.

  • Elective delivery at 32 to 34 weeks’ gestation is recommended if fetal testing remains reassuring and cesarean delivery is most commonly recommended.


Monoamniotic twinning is an unusual form of twinning in which both twins occupy a single amniotic sac. Monoamniotic twins account for 1% of all monozygotic twin pregnancies (Benirschke and Kim, 1973). This form of monozygotic twinning typically occurs when a single embryo splits between the ninth and twelfth day after fertilization. Splitting before this time gives rise to either dichorionic diamniotic twins (split between the first and third day after fertilization) or to monochorionic diamniotic twins (split between the third and eighth day after fertilization). Splitting later in embryogenesis (after the twelfth day postfertilization) gives rise to conjoined twins (Benirschke, 1998).

Although a rare event, monoamniotic twinning is important because of the high perinatal mortality rate associated with these pregnancies. The first comprehensive review of the world literature was performed in 1935 by Quigley, who found an overall mortality rate of 68% in 94 pregnancies. His opinion was that the poor prognosis was due mainly to twisting and knotting of the umbilical cords with subsequent occlusion of the blood supply to one or both twins. A subsequent review in 1959 added 35 new cases to the world literature and reported a high fetal mortality rate of 30% (Salerno, 1959). More recent series and reviews of prenatally diagnosed cases suggest mortality rates ranging from 10% to 32% (Rodis et al., 1997; Allen et al., 2001; Roque et al., 2003; Demaria et al., 2004; Heyborne et al., 2005). This decrease in perinatal mortality is likely secondary to increased rates of prenatal diagnosis, antenatal steroids, intense fetal surveillance, and timed delivery.


It is difficult to ascertain the exact incidence of monoamniotic twins. The incidence has ranged in various studies ...

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