Sometimes, as a result of fertility treatments, a woman becomes pregnant with multiple fetuses. Carrying two fetuses is not considered especially risky, as some women have twins without medical intervention. Even carrying three fetuses is generally not considered especially risky, and triplet births occasionally take place naturally. However, if there are five or more fetuses, there is a significant risk that they will all die or be born prematurely, leading to terrible illnesses.
The vast majority of poskim agree that it is permissible to destroy some of the fetuses so that the others will survive. (The process is euphemistically called “fetal reduction.”) Some explain that each fetus is considered a rodef relative to the others. Others say that because fetuses are not yet considered human life, one may kill some to save the rest. Accordingly, in a triplet pregnancy, only in an unusual case of heightened risk may one kill a fetus. In a quadruplet pregnancy, the situation must be carefully assessed; most of the time it will be permissible to kill one fetus, and two if necessary. In a quintuplet pregnancy, there is a consensus that taking the lives of some of the fetuses is permissible in order to ensure the survival of the rest. Each individual case demands the opinion of a God-fearing doctor and then consultation with a rabbi.
Sometimes, in a twin or triplet pregnancy, one of the fetuses develops more slowly than the others. If the doctor feels that keeping that fetus alive will cause the loss of the other fetuses, it is permissible to take its life in order to save the others. This is both because the weaker fetus would likely not survive in any case and in order to save the remaining fetuses.