Abortion is the loss or failure of an early pregnancy and it is deﬁned in several forms: complete, incomplete, inevitable, missed, septic, and threatened. A complete abortion is the termination of a pregnancy before the age of viability, typically deﬁned as occurring at less than 20 weeks from the ﬁrst day of the last normal menstrual period or involving a fetus of weight less than 500 g. Most complete abortions generally occur before 6 weeks or after 14 weeks of gestation. An incomplete abortion is the spontaneous passage of some, but not all, of the products of conception. A pregnancy in which rupture of the membranes and/or cervical dilation takes place during the ﬁrst half of pregnancy is labeled an inevitable abortion. Uterine contractions typically follow, ending in spontaneous loss of the pregnancy for most patients. A missed abortion is the retention of a failed intrauterine pregnancy for an extended period. A septic abortion is a variant of an incomplete abortion in which infection of the uterus and its contents has occurred. A threatened abortion is a pregnancy that is at risk for some reason. Most often, this applies to any pregnancy in which vaginal bleeding or uterine cramping takes place but no cervical changes have occurred. Estimates for the frequency of complete abortions are as high as 50% to 60% of all conceptions and between 10% and 20% of known pregnancies. Less than 2% of fetal losses are missed abortions. Septic abortions occur in 0.40 to 0.6 of 100,000 spontaneous pregnancy losses. Threatened abortions occur in 30% to 40% of pregnant women.
Abortion may be initiated by the death of the embryo or fetus, followed shortly thereafter by gradual involution of the placenta, which leads to its partial or total separation. Another possibility is that the placental separation may precede the death of the fetus. In either event, the clinical signs and symptoms of abortion manifest themselves with vaginal bleeding followed by expulsive uterine contractions and cervical dilation.
Clinically, a distinction is made between threatened abortion and inevitable abortion. In the former, slight vaginal bleeding is seen, with or without feeble uterine contractions. The characteristic ﬁnding of this type of abortion is the absence of cervical dilation. Inevitable abortion is characterized by cervical dilation together with more severe vaginal bleeding and uterine contractions. The distinction between threatened and inevitable is of some prognostic importance, because in a fair number of cases of threatened abortion, pregnancy can proceed until full viability.
In inevitable abortion, uterine contractions become stronger as time progresses, bleeding becomes more severe, and the process ends by expulsion of the uterine contents. Abortion is called complete when the entire fetus, placenta, and membranes are eliminated. It is called incomplete when the fetus is expelled and all or part of the placenta remains inside the uterus. In the latter case, vaginal bleeding may continue as long as the placental parts are not removed spontaneously or by intervention.
In missed abortion, the fetus (if present) dies but the placenta is not detached from the uterine walls. In such cases the amniotic ﬂuid is reabsorbed, and the fetus undergoes a process of dehydration and mummiﬁcation.
Ultrasonography is useful in establishing the presence of a living embryo. The established tests for pregnancy, determining the presence of chorionic gonadotropins, are usually positive as long as any part of the placental tissue remains in contact with the maternal circulation, and, after complete abortion, until the circulating chorionic gonadotropic hormones are completely eliminated. Although the presence of a living embryo is reassuring, it does not guarantee the successful outcome of the pregnancy.