In 1977, newborn Siamese twins were brought to Children’s Hospital in Philadelphia, where Dr. C. Everett Koop (subsequently the Surgeon General of the United States) was the hospital’s chief of surgery. Physicians had clearly indicated that since the twins were sharing vital internal organs, both would die if remained conjoined. The only option was to operate, thereby killing one and saving the other.
The moral dilemma was set in motion when doctors on the case asked themselves if this procedure constituted murder. A heated debate ensued.

Koop is reported to have taken the initiative and suggested that the issue be referred to a rabbi who was known to be a great scholar and a pious individual. Whatever he said was proper, Koop said, would be the approach taken with the children. The rabbi was Moshe Feinstein, recognized as the greatest Talmudic and legal authorities ever to live in the United States. Rabbi Feinstein asked the doctors how they intended to do the surgery.

They replied, “We will save Baby A, and kill Baby B.”
Rabbi Feinstein then asked, “Could you reverse the procedure and achieve the same results? In other words, could you use the available organs to sustain Baby B and allow Baby A to die?” The doctors answered no, that Baby A was the only one they could save.

Rabbi Feinstein advised them to proceed with the surgery. His decision was based on the Jewish law that if one person is directly threatening to kill another, then it is morally appropriate to thwart the pursuer, even by means of killing him. The law of the pursuer, a rodef, in Hebrew, applies even in the case where the threat to life is unintentional.

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Parry, Aaron (2003-03-04). The Complete Idiot's Guide to the Talmud (p. 298). DK Publishing. Kindle Edition.