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KEY POINTS
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Key Points

  • Congenital malformations occur more commonly in twins as compared with singleton gestations and are an important contributor to the increased perinatal mortality associated with multiple gestations.

  • The incidence of congenital anomalies is thought to be more common in monozygotic compared with dizygotic twin pregnancies.

  • Antenatal ultrasound examination is used to detect malformations in twin pregnancies.

  • The antenatal natural history will depend on the malformation diagnosed, whether or not it is discordant, and thechorionicity of the pregnancy.

  • The management will be influenced by the type of abnormality, whether or not it is concordant, the gestational age when diagnosed, and chorionicity.

  • Counseling of parents depends on the type of abnormality and the prognosis for the anomalous twin as well as on the likely outcome for the normal co-twin.

  • Three management options are available in this situation: expectant management, selective termination of the anomalous fetus, and termination of the entire pregnancy.

  • Expectant management of a twin pregnancy discordant for an abnormality is associated with an increased risk for preterm delivery.

  • The method chosen for selective termination will depend on chorionicity. Intracardiac injection of potassium chloride is safe in dichorionic twins, while cord occlusive techniques are necessary for monochorionic pregnancies.

  • Treatment of neonatal twins with malformations will depend on the particular malformation.

  • The recurrence risk for malformations seen in twin pregnancies will depend on the specific abnormality.

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CONDITION
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When congenital malformations occur in a multiple gestation, management decisions can be difficult for both parents and physicians because the fates of such sibling fetuses are necessarily linked. Given that the incidence of multiple gestations is increasing in industrialized countries, primarily because of assisted reproductive technologies (Jewell and Yip, 1995), the management dilemma for twins with malformations will also inevitably increase. Congenital malformations occur more commonly in twins as compared with singleton gestations and are an important contributor to the increased perinatal mortality associated with twin gestation. Some malformations in twins are inherent to the monozygotic twinning process, such as acardiac twinning, conjoined twins, and twin-to-twin transfusion syndrome. These malformations are discussed separately (see Chapters 119121). Other malformations in twins are deformation abnormalities, such as clubfoot, which result from the crowding of the intrauterine environment.

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INCIDENCE
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Using data from the British Columbia Health Surveillance Registry, the incidence of congenital malformations in twin pairs was estimated at 6% (Schinzel et al., 1979). In this series, the incidence of congenital anomalies was 2.5 times more common in monozygotic twins than in dizygotic twins or singletons. The incidence of chromosomal abnormalities is increased twofold in dizygotic twins as compared with agematched singleton pregnancies, but the incidence of nonchromosomal abnormalities is not increased (Drugan et al., 1996).

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In one series of 1424 twin pairs, 445 pairs were monozygotic, 26 of which (6%) had congenital malformations (Cameron et al., 1983). Even among ...

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