Spontaneous Abortion

Abortion is the spontaneous or induced termination of pregnancy before fetal viability. Because this definition encompasses deliberate pregnancy terminations, some prefer miscarriage to refer to spontaneous pregnancy loss. The term recurrent abortion is used to describe consecutive pregnancy losses that may have a common cause. The duration of gestation or fetal weight that defines abortion is not consistent between organizations. For example, the National Center for Health Statistics, the Centers for Disease Control and Prevention (CDC), and the World Health Organization all define abortion as any pregnancy termination— spontaneous or induced—prior to 20 weeks’ gestation or with a fetus born weighing _500 g. 

These criteria are somewhat self contradictory because the average weight of a normally developed 20-week fetus is 320 g, whereas a birthweight of 500 g is the mean for 22 to 23 weeks (Moore, 1977). Th ere is even more confusion because definitions vary widely according to state laws. Technologic development has also resulted in significant evolution leading to current abortion terminology. Transvaginal sonography (TVS) and precise measurement of serum human chorionic gonadotropin (hCG) concentrations allow identification of extremely early pregnancies as well as distinction between intrauterine and ectopic implantations. Their ubiquitous application to everyday practice has spawned a number of other terms.

For example, it is now possible to distinguish between a chemical and a clinical pregnancy. In another example, an ad hoc international consensus group has proposed definitions to clarify outcomes for pregnancy of unknown location—PUL (Barnhart, 2011). The goal is early verification of an ectopic pregnancy, which has specific management options. Intrauterine pregnancies are then managed depending on evidence for a living fetus. Those that eventuate in an early spontaneous abortion are also termed early pregnancy loss and early pregnancy failure.

As just described, spontaneous first-trimester abortion is interchangeably referred to as miscarriage, early pregnancy loss, and early pregnancy failure. Of these, more than 80 percent occur during the first 12 weeks of pregnancy. At this stage, approximately half result from chromosomal anomaliey. Of those with a fetus, there is a 1.5 male:female gender ratio (Benirschke, 2000).

After 12 weeks, both the abortion rate and the incidence of associated chromosomal anomalies decrease. During the first 3 months of pregnancy, death of the embryo or fetus nearly always precedes spontaneous expulsion. Early death of the conceptus is usually accompanied by haemorrhage into the decidua basalis, followed by necrosis of adjacent tissues. Thus, the embryo fetus detaches, stimulating uterine contractions that result in its expulsion. The intact gestational sac is usually filled with fluid, and a small macerated fetus is found in approximately half of these. In the other half, there is no fetus visible—the so-called blighted ovum. Thus, finding the cause of early miscarriage involves ascertaining the cause of fetal death. This is dissimilar from later pregnancy losses in which the fetus usually does not die before expulsion, and thus other explanations are sought.

The reported incidence of spontaneous abortion varies with the sensitivity of methods used to identify them. In a meticulous investigation of 221 healthy women studied through 707 menstrual cycles, Wilcox and colleagues (1988) identified pregnancies using precise assays for extremely low serum β-hCG concentrations. They reported that 31 percent of pregnancies were lost after implantation. Especially important when considering incidence, two thirds of these early losses were clinically silent.