the expulsion of a fetus from the uterus before it has reached the stage of viability (in human beings, usually about the 20th week of gestation). An abortion may occur spontaneously, in which case it is also called a miscarriage, or it may be brought on purposefully, in which case it is often called an induced abortion.

Spontaneous abortions, or miscarriages, occur for many reasons, including disease, trauma, genetic defect, or biochemical incompatibility of mother and fetus. Occasionally a fetus dies in the uterus but fails to be expelled, a condition termed a missed abortion.

Induced abortions may be performed for reasons that fall into four general categories: to preserve the life or physical or mental well-being of the mother; to prevent the completion of a pregnancy that has resulted from rape or incest; to prevent the birth of a child with serious deformity, mental deficiency, or genetic abnormality; or to prevent a birth for social or economic reasons (such as the extreme youth of the pregnant female or the sorely strained resources of the family unit). By some definitions, abortions that are performed to preserve the well-being of the female or in cases of rape or incest are therapeutic, or justifiable, abortions.

Numerous medical techniques exist for performing abortions. During the first trimester (up to about 12 weeks after conception), endometrial aspiration, suction, or curettage may be used to remove the contents of the uterus. In endometrial aspiration, a thin, flexible tube is inserted up the cervical canal (the neck of the womb) and then sucks out the lining of the uterus (the endometrium) by means of an electric pump.

In the related but slightly more onerous procedure known as dilatation and evacuation (also called suction curettage, or vacuum curettage), the cervical canal is enlarged by the insertion of a series of metal dilators while the patient is under anesthesia, after which a rigid suction tube is inserted into the uterus to evacuate its contents. When, in place of suction, a thin metal tool called a curette (curettage) is used to scrape (rather than vacuum out) the contents of the uterus, the procedure is called dilatation and curettage. When combined with dilatation, both evacuation and curettage can be used up to about the 16th week of pregnancy.

From 12 to 19 weeks the injection of a saline solution may be used to trigger uterine contractions; alternatively, the administration of prostaglandins by injection, suppository, or other method may be used to induce contractions, but these substances may cause severe side effects. Hysterotomy, the surgical removal of the uterine contents, may be used during the second trimester or later. In general, the more advanced the pregnancy, the greater the risk to the female of mortality or serious complications following an abortion.

In the late 20th century a new method of induced abortion was discovered that uses the drug RU 486 (RU-486) (mifepristone), an artificial steroid that is closely related to the contraceptive hormone norethnidrone. RU 486 works by blocking the action of the hormone progesterone, which is needed to support the development of a fertilized egg. When ingested within weeks of conception, RU 486 effectively triggers the menstrual cycle and flushes the fertilized egg out of the uterus.

Whether and to what extent induced abortions should be permitted, encouraged, or severely repressed is a social issue that has divided theologians, philosophers, and legislators for centuries. Abortion was apparently a common and socially accepted method of family limitation in the Greco-Roman world. Although Christian theologians early and vehemently condemned abortion, the application of severe criminal sanctions to deter its practice became common only in the 19th century. In the 20th century such sanctions were modified in one way or another in various countries, beginning with the Soviet Union in 1920 and with Japan and several eastern European and Scandinavian countries in the 1950s. In some countries the unavailability of birth-control devices was a factor in the acceptance of abortion. In the late 20th century China used abortion on a large scale as part of its population-control policy. In the early 21st century some jurisdictions with large Roman Catholic populations, such as Portugal and Mexico City, decriminalized abortion despite strong opposition from the church, while others, such as Nicaragua, increased restrictions on it.

A broad social movement for the relaxation or elimination of restrictions on the performance of abortions resulted in the passing of liberalized legislation in several states in the United States during the 1960s. The U.S. Supreme Court (Supreme Court of the United States) ruled in Wade (Roe v. Wade) (1973) that unduly restrictive state regulation of abortion was unconstitutional, in effect legalizing abortion for any reason for women in the first three months of pregnancy. A countermovement for the restoration of strict control over the circumstances under which abortions might be permitted soon sprang up, and the issue became entangled in social and political conflict. In rulings in 1989 and 1992 a more conservative Supreme Court upheld the legality of new state restrictions on abortion, though it proved unwilling to overturn Roe v. Wade itself. In 2007 the court also upheld a federal ban on a rarely used abortion method known as intact dilation and evacuation.

The public debate of the issue has demonstrated the enormous difficulties experienced by political institutions in grappling with the complex and ambiguous ethical problems raised by the question of abortion. Opponents of abortion, or of abortion for any reason other than to save the life of the mother, argue that there is no rational basis for distinguishing the fetus from a newborn infant; each is totally dependent and potentially a member of society, and each possesses a degree of humanity. Proponents of liberalized regulation of abortion hold that only a woman herself, rather than the state, has the right to manage her pregnancy and that the alternative to legal, medically supervised abortion is illegal and demonstrably dangerous, if not deadly, abortion.