Pregnancy Loss

Frequency and Timing of Pregnancy Loss

Embryos implant 6 days after conception. Physical signs are not generally appreciated until 5 to 6 weeks after the last menstrual period. Fewer than half of preimplantation embryos persist , as witnessed by assisted reproductive technology (ART) success rates rarely exceeding 40% to 50% of in vitro fertilization (IVF) cycles initiated. Even immediately after implantation, determined chemically by the presence of β-human chorionic gonadotropin (β-hCG) in maternal serum, about 30% of pregnancies are lost. After clinical recognition, 10% to 12% are lost. Most clinical pregnancy losses occur before 8 weeks.

Before widespread availability of ultrasound imaging, early fetal demise was often not appreciated until 9 to 12 weeks’ gestation, when bleeding and passage of tissue (products of conception) occurred. With the widespread availability of ultrasound, it has been shown that fetal demise often occurs weeks before overt clinical signs are manifested. This conclusion was reached on the basis of cohort studies showing that only 3% of viable pregnancies are lost after 8 weeks’ gestation. Fetal viability (heart rate activity) may thus cease weeks before maternal symptoms of pregnancy loss. That almost all losses are retained in utero for an interval before clinical recognition means that virtually all losses could be considered “missed abortion or miscarriage.” After the first trimester, pregnancy losses occur at a slower rate. Loss rates are only 1% in women confirmed by ultrasound to have viable pregnancies at 16 weeks.

Two factors influencing clinical pregnancy loss rates are clinically relevant. Firstly, maternal age is positively correlated with pregnancy loss rates ; a 40-year-old woman has twice the risk of a 20-year-old woman. This occurs in euploid as well as aneuploid pregnancies, as discussed later. Secondly, prior pregnancy loss also increases loss rates, but far less than once believed . Among nulliparous women who have never experienced a loss, the likelihood of pregnancy loss is low: 5% in primiparas and 4% in multiparas ( Table 33.1 ). After one loss, the risk of another is increased but does not exceed 30% to 40% even for women with three or more losses. These risks apply not only to those women whose losses were recognized at 9 to 12 weeks’ gestation but also to those whose pregnancies were ascertained in the fifth week of gestation. Of clinical relevance, there is no scientific evidence that women with three losses are etiologically distinct from those with two losses or even one loss. The situation may be different if, rarely, four or more losses have occurred; different etiologic factors may exist in this uncommon subgroup.

The clinical consequence of the above information is that in order to be judged efficacious in preventing recurrent early pregnancy loss (REPL), therapeutic regimens must show success rates substantially greater than 70%. Essentially no therapeutic regimen proposed so far can make this claim.

Placental Anatomic Characteristics of Successful and Unsuccessful Pregnancies

As judged by adult tissue criteria, the human fetus develops in a low oxygen (O ₂ ) environment. Development of the human placenta is modulated heavily by the intrauterine environment. During the first trimester, development takes place in a low O ₂ environment supported by histotrophic nutrition from the endometrial glands. Consequently, the rate of growth of the chorionic sac in normally developing pregnancies is almost invariable across this period and is remarkably uniform between individuals. Toward the end of the first trimester, the intrauterine environment undergoes radical transformation in association with the onset of the maternal arterial circulation and the switch to hemochorial nutrition ( Chapter 1 ). The accompanying rise in intraplacental O ₂ concentration leads to extensive villous remodeling and in particular to the formation of the membranes of the definitive placenta.

The early human gestational sac is designed to minimize the flux of O ₂ from the maternal blood to fetal circulation. In particular, the extravillous trophoblast that migrates inside the uterine decidua and superficial myometrium creates a cellular shell with plugs inside the tip of the uteroplacental arteries. This additional barrier keeps most of the maternal circulation outside the placenta and thus reduces the chemical activity of free O ₂ radicals inside the placenta during most of the first trimester of human pregnancy. In normal pregnancies, the onset of maternal circulation is a progressive phenomenon, starting at about 9 weeks at the periphery and gradually extending toward the center of the placenta. This process correlates closely with the pattern of trophoblast invasion across the placental bed ( Fig. 33.1 ).

In about two-thirds of early pregnancy failures, there is anatomic evidence of defective placentation, which is mainly characterized by a thinner and fragmented trophoblast shell and reduced trophoblast infiltration of the lumen at the tips of the spiral arteries. This is associated with premature onset of maternal circulation throughout the placenta in most cases of miscarriages. These defects are similar in euploid and most aneuploid miscarriages but are more pronounced in complete hydatidiform moles ( Fig. 33.2 ). In vivo ultrasound imaging and histopathologic data indicate that in most early pregnancy losses, the onset of intervillous circulation is premature and widespread, owing to incomplete transformation and plugging of the uteroplacental arteries. In about 80% of missed miscarriages, the onset of maternal placental circulation is both precocious and generalized throughout the placenta. This occurs independently of the karyotype of the conceptus , leading to high O ₂ concentrations too early in pregnancy with secondary widespread trophoblastic oxidative damage and placental degeneration. Although in vitro studies have demonstrated the ability of damaged syncytium to regenerate from the underlying cytotrophoblast, it is likely that in the face of extensive damage this ability will be overwhelmed, leading to complete pregnancy failure.

Numerical Chromosomal Abnormalities

Chromosomal abnormalities are the main cause of both preimplantation and clinically-recognized pregnancy loss. The frequency of losses in human preimplantation embryos is very high. Of morphologically normal embryos, 25% to 50% show chromosomal abnormalities (aneuploidy or polyploidy), depending on maternal age. The frequency of chromosomal abnormalities in morphologically abnormal embryos is even higher. Moreover, these data are based on studies using fluorescence in situ hybridization (FISH) with chromosome-specific probes for only seven to nine chromosomes; rates would be higher using technologies that can assess all chromosomes, such as 24-chromosome FISH, single nucleotide polymorphism, or array comparative genome hybridization (CGH) analysis. The high aneuploidy rate in morphologically normal embryos is consistent with a rate of aneuploidy of 5% to 10% in sperm of ostensibly normal men and of 20% in oocytes (deduced from polar bodies) of women undergoing ART. Aneuploidy rates in oocytes and embryos predictably increase as maternal age increases.

At least 50% of clinically recognized pregnancy losses result from a chromosomal abnormality. The frequency is probably higher because if one analyzes chorionic villi recovered by chorionic villus sampling (CVS) immediately after ultrasound diagnosis of fetal demise or during surgical curettage (rather than culturing spontaneously expelled products, which has a lower success rate), the chromosomal abnormalities are detected in 65% to 90% of the products. In addition, microarray-based comparative genomic hybridization analysis can detect subtle abnormalities not evident by karyotype.

Approximately 25% of second-trimester losses have been attributed to genetic etiologies. The most common cytogenetic abnormalities are similar to those seen in live-born infants and include trisomy 21, trisomy 18, and trisomy 13, monosomy X and sex chromosomal polysomy. Cytogenetic abnormalities are more common when fetal structural anomalies are identified. Overall, the frequency of aneuploidies is less than that observed in first-trimester miscarriages but much higher than that found among live-born infants (0.6%).

Types of Numerical Chromosomal Abnormalities

Autosomal Trisomy

Autosomal trisomies represent the largest (about 50%) single class of chromosomal complements in cytogenetically early pregnancy failure . That is, 25% of all miscarriages are trisomic, given half of all abortuses have a chromosomal abnormality. Frequencies of various trisomies are listed in Table 33.2 . Trisomy for every chromosome has been observed. The most common trisomy in first-trimester losses is trisomy 16. Most trisomies show a maternal age effect, but the effect varies markedly among chromosomes. The increased maternal age effect is especially impressive for double trisomies.

TABLE 33.2

Chromosomal Completion in Spontaneous Abortions Recognized Clinically in the First Trimester

Data from Simpson JL, Bombard AT. Chromosomal abnormalities in spontaneous abortion: frequency, pathology and genetic counseling. In: Edmonds K, ed. Spontaneous Abortion . London: Blackwell; 1987.

Chromosomal Complement	Frequency	Percent
Normal 46,XX or 46,XY		54.1
Triploidy		7.7
69,XXX	2.7
69,XYX	0.2
69,XXY	4.0
Other	0.8
Tetraploidy		2.6
92,XXX	1.5
92,XXYY	0.55
Not stated	0.55
Monosomy X		18.6
Structural abnormalities		1.5
Sex chromosomal polysomy		0.2
47,XXX	0.05
47,XXY	0.15
Autosomal monosomy (G)		0.1
Autosomal trisomy for chromosomes		22.3
1	0
2	1.11
3	0.25
4	0.64
5	0.04
6	0.14
7	0.89
8	0.79
9	0.72
10	0.36
11	0.04
12	0.18
13	1.07
14	0.82
15	1.68
16	7.27
17	0.18
18	1.15
19	0.01
20	0.61
21	2.11
22	2.26
Double trisomy		0.7
Mosaic trisomy		1.3
Other abnormalities or not specified		0.9
		100.0

Trisomies incompatible with life predictably show slower growth than trisomies compatible with life (e.g., trisomies 13, 18, 21), but otherwise there are usually no features distinguishing the two groups. Fetuses from the former group may show anomalies consistent with those found in full-term live-born trisomic neonates. Malformations present have been said to be more severe than those observed in induced abortuses following prenatal diagnosis.

Aneuploidy usually results from errors at meiosis I, specifically maternal meiosis I. Errors of maternal meiosis I are associated with advanced maternal age. Once thought to involve mostly mis-segregation of whole chromosomes, it is now clear that chromatid errors are an equally prevalent cause of maternal meiotic errors. Irrespective, the cytologic mechanism involves decreased or absent meiotic recombination. In trisomy 13 and trisomy 21, 90% of these maternal cases arise at meiosis I. Almost all trisomy 16 cases arise in maternal meiosis I. An exception is trisomy 18, in which two thirds of the 90% of maternal meiotic cases arise at meiosis II.

A practical consequence of these data is that deducing chromosomal status of oocytes by analysis of polar bodies can detect 95% of more of chromosomally abnormal embryos. This is relevant because polar body analysis is a more robust predictor of embryo status than analysis of blastomere from a 3-day-old embryo. In the latter, mitotic nondisjunction can lead to spurious and unrepresentative results. Errors in paternal meiosis account for 10% of acrocentric (13, 14, 15, 21, and 22) trisomies . In trisomy 21, paternal meiotic errors are equally likely to arise in meiosis I or II, a circumstance that contrasts with the situation in maternal meiotic errors. Among nonacrocentric chromosomes, paternal contribution is uncommon. A surprising exception involves trisomy 2.

Polyploidy

In polyploidy, more than two haploid chromosomal complements are present. Nonmosaic triploidy (3 n = 69) and tetraploidy (4 n = 92) are common in miscarriage products . This phenomenon is presumably distinct from the diploid or triploid mosaicism that is found in about 30% of blastocysts. Triploid miscarriages are usually 69,XXY or 69,XXX, resulting from dispermy (paternally inherited). An association exists between these diandric triploidy and the development of partial hydatidiform moles associated with persistent trophoblastic disease; thus, in these cases, appropriate hCG follow-up should be recommended. The more common “complete” (classic) hydatidiform mole is 46,XX, androgenetic in origin, and composed exclusively of villous tissue. Pathologic findings in diandric triploid and tetraploid placentas include a disproportionately large gestational sac, focal (partial) hydropic degeneration of placental villi, and trophoblast hyperplasia. Placental hydropic changes are progressive and may be difficult to identify in early pregnancy. By contrast, placental villi often undergo hydropic degeneration after fetal demise. This can occur in all types of miscarriage and thus, histologic and cytogenetic investigations are essential to differentiate between true mole and pseudo-mole because only a true mole can be associated with persistent trophoblastic disease. Fetal malformations associated with triploid miscarriage include neural tube defects and omphaloceles, anomalies reminiscent of those observed in triploid conceptuses surviving to term. Facial dysmorphia and limb abnormalities have also been reported. Tetraploidy is uncommon, rarely progressing beyond 2 to 3 weeks of embryonic life. This chromosomal abnormality can also be associated with persistent trophoblastic disease and thus needs to be identified in order to offer hCG follow-up.

Sex Chromosomal Polysomy (X or Y)

The complements 47,XXY and 47,XYY each occur in about 1 per 800 live-born male births; 47,XXX occurs in 1 per 800 female births . X or Y polysomy are only slightly more common in miscarriages than in live-born infants. In pregnancies conceived by intracytoplasmic sperm injection, the frequency of 47,XXX and 47,XXY embryos and fetuses appears to be increased.

Monosomy X

Monosomy X is the single most common chromosomal abnormality among miscarriages, accounting for 15% to 20% of abnormal specimens. Monosomy X embryos usually consist of only an umbilical cord stump. Later in gestation, anomalies characteristic of Turner syndrome may be seen, such as cystic hygromas and generalized edema ( Fig. 33.3 ). Unlike live-born 45,X individuals, 45,X abortuses show germ cells; however, germ cells rarely develop beyond the primordial germ cell stage. The pathogenesis of 45,X germ cell failure thus involves not so much failure of germ cell development as more rapid attrition in 45,X compared with 46,XX embryos. Monosomy X usually (80%) occurs as result of paternal sex chromosome loss. This observation is consistent with the lack of a maternal age effect in 45,X.

Chromosomal Rearrangements

Mendelian and Polygenic/Multifactorial Etiology

Even in the 30% to 50% of first-trimester pregnancy losses that show no chromosomal abnormalities, genetic causation is not excluded. Fetal demise may have occurred as a result of other genetic etiologies. Specifically, neither mendelian nor polygenic/multifactorial disorders show chromosomal abnormalities, and yet these etiologies explain far more congenital anomalies in live-borns than do chromosomal abnormalities. Mendelian and polygenic/multifactorial factors must therefore play a pivotal role in embryonic mortality and there are innumerable candidate genes. Especially likely to be mendelian or polygenic in etiology are early pregnancy losses that demonstrate isolated structural anomalies. Cytogenetic data are often lacking in these specimens, making it nearly impossible to determine the relative role of cytogenetic versus mendelian or polygenic mechanisms in early embryonic maldevelopment. Transcervical embryoscopy prior to curettage correlated with cytogenetic analysis of chorionic villous tissue in missed miscarriages has shown that embryos with chromosomal abnormalities usually showed one or more external anomalies but also that 18% had a morphologic defect with a normal karyotype.

In addition to traditional single-gene perturbations (mendelian etiology), novel nonmendelian forms of inheritance play a greater role in embryonic loss. Mosaicism may be restricted to the placenta, the embryo being normal per se. This phenomenon is termed confined placental mosaicism . Losses caused by this mechanism may already be subsumed in extant data because most studies involved analysis only of villous material. A corollary of confined placental mosaicism is uniparental disomy, in which both homologues for a given chromosome are derived from a single parent. This presumably occurs as result of expulsion of a chromosome from a trisomic zygote (“trisomic rescue”). Although the karyotype would appear normal (46,XX or 46,XY), the product would lack a contribution from one parent.

Luteal Phase Defects

Implantation in an inhospitable endometrium is a plausible explanation for pregnancy loss. Progesterone deficiency in particular could result in the estrogen-primed endometrium being unable to sustain implantation. Luteal phase deficiency (LPD) has long been hypothesized, specifically caused by inadequate progesterone secreted by the corpus luteum. Yet, endometrial histology identical to that observed with luteal phase “defects” is observed in fertile women and there is no convincing evidence that LPD is associated with infertility and REPL.

A recent multicenter, double-blind, placebo-controlled, randomized trial investigated whether treatment with progesterone would increase the rates of live births among women with unexplained recurrent miscarriages. A group of 836 women who conceived naturally were randomly assigned to receive either progesterone (404 women) or placebo (432 women). There were no significant between-group differences in the rates of live births or of adverse events.

Luteal phase abnormalities arising during ovulation stimulation and oocyte retrieval as necessitated during ART could be a different phenomenon. Progesterone supplementation during the luteal phase is associated with higher rates of live birth or ongoing pregnancy than placebo in this setting. It is considered standard to administer progesterone until about 9 weeks’ gestation. In this circumstance, the cells surrounding the oocyte, which would ordinarily contribute to the corpus luteum, may have been removed when the oocyte was aspirated.