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  • The incidence of multiple gestations has risen significantly over several decades, primarily due to increased use of fertility drugs for ovulation induction, superovulation, and assisted reproductive technologies (ART), such as in vitro fertilization (IVF).
  • Compared with singleton pregnancies, twin pregnancies are more likely to be complicated by hypertensive disorders, gestational diabetes mellitus, anemia, preterm birth, ante- and postpartum hemorrhage, and maternal death.
  • The perinatal mortality rate of twins is 3–4 times higher—and for triplets much higher still—than in singleton pregnancies as a result of chromosomal abnormalities, prematurity, structural anomalies, hypoxia, and trauma.
  • This is particularly true of monozygotic twins, which are also uniquely susceptible to twin–twin transfusion syndrome.

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In the United States, between 1980 and 2004, the twin rate climbed 101% with 68,339 twins born in 1980 and 137,085 twins born in 2006. Multiple gestations now comprise 3% of all pregnancies, and twins comprise 25–30% of deliveries resulting from assisted reproductive technologies (ART). Significant maternal and neonatal effects are felt from this increase in multiple births. The financial costs are also staggering, with combined costs of ART plus pregnancy care, delivery, and neonatal care reaching hundreds of thousands of dollars in some cases. Maternal morbidity and mortality rates are much higher in multiple pregnancy than in singleton pregnancy. Compared with singleton pregnancies, twin pregnancies are more likely to be complicated by hypertensive disorders, gestational diabetes mellitus, anemia, preterm birth, ante- and postpartum hemorrhage, and maternal death. Earlier and more precise sonography has revealed the incidence of multiple gestation to be 3.29–5.39% before 12 weeks. However, in over 20% of such cases, one or more of the pregnancies spontaneously disappears (“vanishing twin”). Although this event may be associated with vaginal bleeding, the prognosis remains good for the remaining twin.

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Approximately two-thirds of twin pregnancies end in a singleton birth; the other embryo is lost from bleeding, is absorbed within the first 10 weeks of pregnancy, or is retained and becomes mummified (fetus papyraceous). Fetus papyraceous is a small, blighted, mummified fetus usually discovered at the delivery of a well-developed newborn. This occurs once in 17,000–20,000 pregnancies spontaneously and is also the result of multifetal reduction. The cause is thought to be death of one twin, amniotic fluid loss, or reabsorption and compression of the dead fetus by the surviving twin.

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Twins can be monozygotic or dizygotic. Higher order multiples can result from either or both processes.

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Monozygotic Multiple Gestation

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Monozygotic twins (“identical twins”) are the result of the division of a single fertilized ovum that subsequently divides into 2 separate individuals. Monozygotic twinning occurs in about 4–5 of 1000 pregnancies in all races. The rate is remarkably constant in all populations and is not influenced by heredity, age of the mother, or other factors. Monozygotic twins are always of the same sex. However, the twins may develop differently depending on the time of preimplantation division. Normally, monozygotic twins share the same physical ...

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