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Abortion of one or more (but not all) fetuses in a multiple pregnancy
[[File:UOTW 19 Ultrasound of the Week 1. webm|thumb|Monoamniotic triplets, a very rare condition where the triplets share a single placenta, seen on ultrasound. Because a single placenta has difficulty supporting multiple fetuses, in such cases a selective reduction to improve the likelihood of survival for the remaining fetus or fetuses may be indicated. The procedure is most commonly done to reduce the number of fetuses in a multiple pregnancy to a safe number, when the multiple pregnancy is the result of use of assisted reproductive technology; outcomes for both the mother and the babies are generally worse the higher the number of fetuses. The procedure is also used in multiple pregnancies when one of the fetuses has a serious and incurable disease, or in the case where one of the fetuses is outside the uterus, in which case it is called selective termination. Risks of the procedure include bleeding requiring transfusion, rupture of the uterus, retained placenta, infection, a miscarriage, and prelabor rupture of membranes. Each of these appears to be rare.

thumb|TRAP syndrome. On the left, an acardiac twin which lacks both a heart and head. On the right, the "pump twin" which supplies the acardiac twin with blood. A selective termination of the acardiac twin reduces the risk of death of the pump twin. Selective reduction is used when a mother is carrying an unsafe or undesirable number of fetuses in a multiple pregnancy, which are common in medically assisted pregnancies. The result is a reduction in the number of fetuses to a number that is more safe for the mother and the remaining fetuses or more compatible with socio economic constraints on the caregivers. and when one or more of the fetuses has a serious and incurable disease. While the data is weak, due to the small sizes of studies and the lack of randomized controlled trials, as of 2017 it appeared that when short term perinatal outcomes in multiple pregnancies reduced to twins are compared to those of non reduced triplets, there were fewer deaths among the babies born to mothers who underwent reduction, the twins were born later and were less likely to be premature, and had higher birthweight.

Generally selective reduction reduces the risk of preterm birth, leading to better outcomes for both mothers and the newborns. It appears that reduction of triplets, where each triplet is in its own placenta, to twins results in a lower risk of preterm birth and does not increase the risk of miscarriage. In triplets where two of the fetuses share a placenta and each has its own amniotic sac, it appears, with less certainty, that there is also a lower risk of preterm birth and no increase in the risk of miscarriage.

Risks of the procedure include bleeding requiring transfusion, rupture of the uterus, retained placenta, infection, a miscarriage, and prelabor rupture of membranes. Each of these appears to be rare. Over time, more and more women in the developed world sought to become pregnant when they were older, having the first child when they were over forty years old. At the same time, the number of obstetrician gynecologists with the required expertise increased and their role in family planning evolved. These trends led to more women asking for reduction to one fetus. These requests increasingly came from women pregnant with twins due to advances in the field of ART which made massively multiple pregnancies rarer. Selective reduction of twins was very controversial at first but has gradually become more accepted. One of the authors of the 1988 guidelines updated his stance and expressed a desire for the procedure to become more available in 2014. Sex selective reduction is widely considered to be unethical in making decisions about which fetus to keep.