Chromosomal Abnormalities and Embryonic Phenotype

Abstract

Pregnancy loss during the embryonic period of development is a common event. Among the estimated 15 to 20% of clinically recognized pregnancies that are lost, the majority take place when the conceptus is undergoing embryonic development. The abnormal morphology of aborted embryos has been noted by earlier authors (Mall, 1908; Hertig et al., 1936). It has since been recognized that the incidence of cytogenetic abnormalities among such embryonic spontaneous abortions is over 50% (Boué et al., 1975; Jacobs and Hassold, 1987), with chromosomal trisomy, monosomy, and polyploidy being the most common. Nearly all these chromosomal defects are “de novo,” arising either during parental gametogenesis or fertilization/ cleavage. Trisomies and monosomies are caused by meiotic chromosomal nondisjunction; triploidy usually results from double fertilization of an ova by two sperms and tetraploidy from abnormal cleavage. Many studies have been dedicated to answering the question whether an abortion with an identified chromosomal defect—specifically, chromosomal trisomy—predisposes the parents to the higher risk of having a liveborn infant with chromosomal trisomy. When allowance was made for maternal age, no association between spontaneous abortion with aneuploidy and an increased risk of a future liveborn infant with a chromosomal defect could be demonstrated (Warburton et al., 1987).