Abortion is the spontaneous or induced termination of pregnancy before fetal viability. Thus, miscarriage and abortion appropriately are terms used interchangeably in a medical context. But, because popular use of abortion by laypersons implies a deliberate intact pregnancy termination, many prefer miscarriage for spontaneous fetal loss. Both terms will be used throughout this chapter.
For statistical and legal purposes, viability is usually defined by pregnancy duration and fetal birthweight. The Centers for Disease Control and Prevention (CDC) and the World Health Organization (WHO) define abortion as any pregnancy termination—spontaneous or induced—before 20 weeks’ gestation or with a fetus born weighing <500 g. Confusion may be introduced by individual states whose laws define abortion more widely.
Technologic developments have revolutionized current abortion terminology. Transvaginal sonography (TVS) and precise measurement of serum beta human chorionic gonadotropin (β-hCG) concentrations help to identify extremely early pregnancies and to clarify intrauterine versus ectopic location. The term pregnancy of unknown location (PUL) aids timely identification and management of ectopic pregnancy. In the context of early pregnancies, five categories have been proposed: definite ectopic pregnancy, probable ectopic, PUL, probable intrauterine pregnancy (IUP), and definite IUP (Barnhart, 2011). Management options for ectopic pregnancies are described in Chapter 7 (p. 168). Of IUPs, those that spontaneously abort during the first trimester, that is, within the first 126/7 weeks of gestation, are also defined as early pregnancy loss by the American College of Obstetricians and Gynecologists (2018a). Recurrent pregnancy loss is variably defined but is meant to identify women with repetitive miscarriage.
If studies are confined to first-trimester abortions, up to 70 percent are anembryonic, that is, with no identifiable embryonic elements (Du, 2018; Soler, 2017). The previous term blighted ovum is no longer preferred. The remaining pregnancies are embryonic miscarriages, which may be further grouped as either those with chromosomal anomalies—aneuploid abortions, or those with a normal chromosomal complement—euploid abortions.
More than 80 percent of spontaneous abortions occur during the first 12 weeks of gestation (American College of Obstetricians and Gynecologists, 2018a). Their rate increases significantly with advancing maternal age. For example, the clinical miscarriage rate nearly doubles with maternal or paternal age older than 40 (Gracia, 2005; Kleinhaus, 2006).
The reported incidence of spontaneous abortion varies with the sensitivity of methods used to identify them. Wilcox and colleagues (1988) studied 221 healthy women through 707 menstrual cycles. They used highly specific assays sensitive to minute concentrations of maternal serum β-hCG and found that 31 percent of pregnancies were lost after implantation. Importantly, two thirds of these very early losses were clinically silent.
Approximately half of embryonic first-trimester miscarriages are aneuploid. This incidence declines markedly with advancing gestation ...