Abortion, termination of pregnancy before the fetus is capable of independent life. When the expulsion from the womb occurs after the fetus becomes viable (capable of independent life), usually at the end of six months of pregnancy, it is technically a premature birth.

Types of Abortions
Abortion may be spontaneous or induced. Expelled fetuses weighing less than 0.50 kg or of less than 20 weeks' gestation are usually considered abortions.

Spontaneous
It is estimated that some 25 percent of all human pregnancies terminate spontaneously in abortion, with three out of four abortions occurring during the first three months of pregnancy. Some women apparently have a tendency to abort, and recurrent abortion decreases the probability of subsequent successful childbirth.
The causes of spontaneous abortions, or miscarriages, are not clearly established. Abnormal development of the embryo or placental tissue, or both, is found in about half the cases; these abnormalities may be due to inherent faults in the germ cells or may be secondary to faulty implantation of the developing ovum or to other characteristics of the maternal environment. Severe vitamin deficiencies have been shown to play a role in abortions in experimental animals. Hormone deficiencies also have been found in women who are subject to recurrent abortions. Spontaneous abortions may be caused also by such maternal abnormalities as acute infectious diseases, systemic diseases such as nephritis and diabetes, and severe trauma. Uterine malformations, including tumors, are responsible in some instances, and extreme anxiety and other psychic disturbances may contribute to the premature expulsion of the fetus. The most common symptom of threatened abortion is vaginal bleeding, with or without intermittent pain. About one-fourth of all pregnant women bleed at some time during early pregnancy, however, and up to 50 percent of these women carry the fetus for the full term. Treatment for threatened abortion usually consists of bed rest. Almost continuous bed rest throughout pregnancy is required in some cases of repeated abortion; vitamin and hormone therapy also may be given. Surgical correction of uterine abnormalities may be indicated in certain of these cases.
Spontaneous abortion may result in expulsion of all or part of the contents of the uterus, or the embryo may die and be retained in the uterus for weeks or months in a so-called missed abortion.