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.