The word abortion derives from the Latin aboriri—to miscarry. According to the New Shorter Oxford Dictionary (2002), abortion is premature birth before a live birth is possible, and in this sense it is synonymous with miscarriage. It also means an induced pregnancy termination to destroy the fetus. Although both terms are used interchangeably in a medical context, popular use of the word abortion by laypersons implies a deliberate pregnancy termination. Thus, many prefer miscarriage to refer to spontaneous fetal loss before viability. To add to confusion, widespread use of sonography and measurement of serum human chorionic gonadotropin levels allow identification of extremely early pregnancies along with terms to describe these. Some examples are early pregnancy loss or early pregnancy failure. Throughout this book, we employ all of these at one time or another.
The duration of pregnancy is also used to define and classify abortions for statistical and legal purposes (see Chap. 1, Definitions). For example, the National Center for Health Statistics, the Centers for Disease Control and Prevention, and the World Health Organization define abortion as pregnancy termination prior to 20 weeks' gestation or with a fetus born weighing less than 500 g. Despite this, definitions vary widely according to state laws.
More than 80 percent of spontaneous abortions are in the first 12 weeks. As shown in Figure 9-1, at least half result from chromosomal anomalies. There also appears to be a 1.5 male:female gender ratio in early abortuses (Benirschke and Kaufmann, 2000). After the first trimester, both the abortion rate and the incidence of chromosomal anomalies decrease.
Frequency of chromosomal anomalies in abortuses and stillbirths during each trimester. Approximate percentages for each group are shown. (Data from Eiben, 1990; Fantel, 1980; Warburton, 1980, and all their colleagues.)
Hemorrhage into the decidua basalis, with adjacent tissue necrosis, usually accompanies early miscarriage. In these cases, the ovum detaches, and this stimulates uterine contractions that result in expulsion. When a gestational sac is opened, fluid is commonly found surrounding a small macerated fetus, or alternatively, there is no fetus—the so-called blighted ovum.
The prevalence of spontaneous abortion varies according to diligence used in its identification. For example, Wilcox and colleagues (1988) studied 221 healthy women through 707 menstrual cycles. They found that 31 percent of pregnancies were lost after implantation. Importantly, using highly specific assays for minute concentrations of maternal serum β-human chorionic gonadotropin (β-hCG), two thirds of these early losses were designated as clinically silent.
A number of factors influence the spontaneous abortion rate, but it is not known at this time if those that are clinically silent are affected by some of these. For example, clinically apparent miscarriage increases with parity as well as with maternal and paternal age (Gracia, 2005; Warburton, 1964; Wilson, 1986, and all their colleagues). The frequency doubles from 12 percent in women younger than 20 years to 26 percent in those older than 40 years. For the same comparison of paternal ages, the frequency increases from 12 to 20 percent. But again, it is not known if clinically silent miscarriages are similarly affected by age and parity.
Although mechanisms responsible for abortion are not always apparent, during the first 3 months of pregnancy, death of the embryo or fetus nearly always precedes spontaneous expulsion. Thus, finding the cause of early abortion involves ascertaining the cause of fetal death. In later losses, the fetus usually does not die before expulsion, and other explanations are sought.
Early spontaneous abortions commonly display a developmental abnormality of the zygote, embryo, fetus, or at times, the placenta. Of 1000 spontaneous abortions analyzed by Hertig and Sheldon (1943), half had a degenerated or absent embryo—the blighted ovum described previously. In 50 to 60 percent of spontaneously aborted embryos and early fetuses, abnormalities in chromosomal numbers account for most wastage (Table 9-1). Chromosomal errors become less common with advancing pregnancy and are found in approximately a third of second-trimester losses but in only 5 percent of third-trimester stillbirths (see Chap. 29, Fetal Death).
Table 9-1. Chromosomal Findings in Abortuses
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Table 9-1. Chromosomal Findings in Abortuses
Incidence in Percent
Kajii et al. (1980)
Eiben et al. (1990)
46,XY and 46,XX
Monosomy X (45,X)
Double or triple trisomy
Approximately 95 percent of chromosomal abnormalities are caused by maternal gametogenesis errors, whereas 5 percent are due to paternal errors (Jacobs and Hassold, 1980). Those found most commonly in abortuses are listed in Table 9-1.
Autosomal trisomy is the most frequently identified chromosomal anomaly with first-trimester miscarriages. Although most trisomies result from isolated nondisjunction, balanced structural chromosomal rearrangements are present in one partner in 2 to 4 percent of couples with recurrent miscarriage (American College of Obstetricians and Gynecologists, 2001). Autosomal trisomies for all except chromosome number 1 have been identified in abortuses, and those with autosomes 13, 16, 18, 21, and 22 are most common. Bianco and colleagues (2006) recently described that a previous miscarriage increased the risk of a subsequent fetal aneuploidy from a baseline risk of 1.39 percent to 1.67 percent in almost 47,000 women. Two or three previous miscarriages increased this to 1.84 and 2.18 percent, respectively.