Nondisjunction creates an equal number of nullisomic and disomic gametes. As a rule, missing chromosomal material is more devastating than extra chromosomal material, and almost all monosomic conceptuses are lost before implantation. he one exception is monosomy for the X chromosome (45,X), Turner syndrome, which is discussed subsequently. Despite the strong association between maternal age and trisomy, maternal age and monosomy are not linked.
This is an abnormal number of complete haploid chromosomal sets. Polyploidy accounts for approximately 20 percent of spon taneous abortions but is rare in later gestations.
Triploid pregnancies have three haploid sets-69 chromosomes. One parent must contribute two sets, and the phenotypic presentation difers according to the parent of origin. With diandric triploidy, also known as type I triploidy, the extra chromosomal set is paternal, resulting from fertilization of one egg by two sperm or by a single diploid-and thus abnormal-sperm. Diandric triploidy produces a partial molar pregnancy, discussed in Chapter 20 (p. 389). Diandric triploidy accounts for most triploid conceptions, but their first-trimester loss rate is extremely high. As a result, two thirds of triploid pregnancies identified beyond the first trimester are caused instead by diynic triploidy Q auniaux, 1999). With a digynic triploid pregnancy, also known as type II triploidy, the extra chromosomal set is maternal, and the egg fails to undergo the irst or second meiotic division before fertilization. Digynic triploid placentas do not develop molar changes. However, the fetus usually displays asymmetrical growth restriction.
he prevalence of recognized triploid pregnancies approximates 1 in 5000 pregnancies (Zalel, 2016). Triploidy is a lethal aneuploidy, and more than 90 percent of fetuses with either the diandric or digynic form have multiple structural anomalies. hese include central nervous system defects-particularly involving the posterior fossa, as well as cardiac, renal, and extremity anomalies Qauniaux, 1999; Zalel, 2016). Counseling, prenatal diagnosis, and delivery options are similar to those for trisomies 18 and 13. he recurrence risk for a woman whose triploid fetus survived past the first trimester is 1 to 1.5 percent, and thus prenatal diagnosis is ofered in future pregnancies (Gardner, 1996).
Tetraploid pregnancies have four haploid sets of chromosomes, resulting in either 92,XXor 92,YY. This suggests a postzygotic failure to complete an early cleavage division. The conceptus invariably succumbs, and the recurrence risk is minimal.
45, X-Turner Syndrome. First described by Turner (1938), this syndrome later was found to be caused by monosomy X (Ford, 1959). he prevalence of Turner syndrome is approximately 1 in 2500 liveborn girls (Cragan, 2009; Dolk, 2010). The missing X chromosome is paternally derived in 80 percent of cases (Cockwell, 1991; Hassold, 1990). Screening for Turner syndrome with cell-free DNA is discussed in Chapter 14 (p. 284).