While identical (monozygotic) twins are much less common than dizygotic ones, monozygotic twinning events are increased after induced ovulation and in vitro fertilization. Monozygotic twins can be diamniotic dichorionic (two amniotic sacs, two placentas), monoamniotic monochorionic (one amniotic sac, one placenta), or diamniotic monochorionic (two amniotic sacs, one placenta). The last type accounts for approximately two-thirds of all monozygotic twins, and most of those are without overt complications.
Fisk and colleagues have studied records of 151 such seemingly uncomplicated diamniotic monochorionic pregnancies and found a surprisingly high rate of fetal death: ten unexpected intrauterine deaths occurred in seven of the 151 pregnancies with no prior signs of complications. All deaths occurred within two weeks of a normal scan, at a median gestational age of 34 weeks and 1 day.
The authors conclude that "despite intensive fetal surveillance, structurally normal monochorionic diamniotic twin pregnancies are complicated by a high rate of intrauterine death." As the deaths occurred predominantly after 32 weeks' gestation, the authors suggest that the prospe
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