Rates for miscarriage vary according to the study population. In pregnancies aged 5 to 20 weeks' gestation, the incidence ranges from 11 to 22 percent and is higher in earlier weeks (Ammon Avalos, 2012). To evaluate rates starting at conception, Wilcox and colleagues (1988) studied 221 healthy women trying to conceive through 707 menstrual cycles and found a miscarriage rate of 31 percent. This study found that two thirds of these losses are early and clinicaly silent. Currently, certain factors are known to inluence clinicaly apparent miscarriage. However, it is unknown if these same factors also afect clinically silent pregnancy loss.
Of all miscarriages, approximately half are euploid abortions, that is, carrying a normal chromosomal complement. The other half has a chromosomal abnormality. Initially determined by tissue karyotyping, this percentage appears to persist even when implementing newer cytogenetic techniques Qenderny, 2014). Notably, the American College of Obstetricians and Gynecologists (20 16d) does not recommend routine use of chromosomal microarray testing of irst-trimester fetal tissues. However, these organizations, and the American Society for Reproduc tive Medicine (2012), recognize its value if cytogenetic analysis alters future care.
Both abortion and chromosomal anomaly rates decline with advancing gestational age (Ammon Avalos, 2012; Eiben, 1990).
Kajii and associates (1980) noted that 75 percent of chromo somally abnormal abortions occurred by 8 weeks' gestation. Of chromosomal abnormalities, 95 percent are caused by maternal gametogenesis errors, and 5 percent by paternal errors Qacobs, 1980). Most common abnormalities are trisomy, found in 50 to 60 percent; monosomy X, in 9 to 13 percent; and triploidy, in 11 to 12 percent (Eiben, 1980; Jenderny, 2014).
Trisomies typically result from isolated nondisjunction, rates of which rise with maternal age (Bow§, 1975). Trisomies of chromosomes 13, 16, 18, 21, and 22 are most common. In contrast, balanced structural chromosomal rearrangements may originate from either parent and are found in 2 to 4 percent of couples with recurrent pregnancy loss.
Monosomy X (45,X) is the single most frequent specific chro mosomal abnormality. his is Turner syndrome, which usually results in abortion, but liveborn females are described in Chap ter 13 (p. 259). Conversely, autosomal monosomy is rare and incompatible with life.
Triploidy is often associated with hydropic or molar placental degeneration (Chap. 20, p. 389). The fetus within a partial hydatidiform mole frequently aborts early, and the few carried longer are all grossly deformed. Advanced maternal and paternal ages do not increase the incidence of triploidy. Tetraploid fetuses most often abort early in gestation, and they are rarely liveborn.
In chromosomally normal pregnancy losses, maternal influences play a role. The causes of euploid abortions are poorly understood, but various medical disorders, environmental conditions, and developmental abnormalities have been implicated.
Euploid pregnancies abort later than aneuploid ones. Specifically, the rate of euploid abortion peaks at approximately 13 weeks (Kajii, 1980). In addition, the incidence of euploid abortion rises dramatically after maternal age exceeds 35 years (Stein, 1980).