SELECTIVE TERMINATION
The incidence of fetal malformations is known to be higher in multifetal pregnancies compared to singletons.33 In dizygotic twins, the risk of malformation is slightly more than twice the risk per pregnancy (independent probabilities per fetus). In monozygotic twins, Mendelian and chromosomal abnormalities are comparable to singletons, while the definite higher risk of structural malformations exists. In nearly 85% of cases irrespective of chorionicity, the malformation is confined to one fetus. After the prenatal diagnosis, a couple would have three options: expectant management, terminate both normal and abnormal fetuses, or ST of the abnormal fetus.
Aberg34 reported the first successful selective birth from a twin pregnancy discordant for Hurler syndrome in 1978. Kerenyi and Chitkara35 followed with a report of ST for twins discordant for Down syndrome in 1981. Throughout the 1980s a number of small series of second-trimester selective terminations appeared in the literature, most of which showed very high loss rates and morbidity.36-38 The high loss rates represented were ultimately determined to be principally in monochorionic pregnancies.
Selective termination refers to an intervention in which one or more specific fetuses with a known or suspected fetal chromosomal, structural, or genetic abnormality of a multifetal gestation are terminated.
ST was initially offered as an option to mothers with fetuses with major nonlethal anomalies that could otherwise lead to the live birth and long-term survival of a severely impaired child. ST is now also offered for fetuses with lethal anomalies because some mothers find it emotionally challenging to carry on baby with a gross fetal anomaly like anencephaly that will die before or shortly after birth. Secondly associated antenatal issues as polyhydramnios or hydrops may increase the risk of PTB.
The main variable in selecting the technique of ST is the chorionicity. In dichorionic (DC) twins, intracardiac KCl in the affected fetus is safe for the normal co-twin. In monochorionic(MC) pregnancies, the risk of passage of KCl into the circulation of the normal co-twin through placental anastomosis precludes this technique. Hence, the accurate determination of chorionicity between 9-13 weeks is crucial before performing ST.