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The theoretical rate of chromosomally abnormal embryos at fertilization is approximately 40 percent, taking into account both the rate of aneuploidies in oocytes and sperm and fertilization‐related abnormalities.^{95, 96} Mouse data show that most aneuploidies, although compatible with cleavage, are lost during implantation.^{97, 98} Additional losses of chromosomally abnormal embryos occur after implantation, clinically recognized as spontaneous abortions, more than half of which are caused by chromosomal abnormalities. As a result of this selection against chromosomal abnormalities before and after implantation, only 0.65 percent of newborns have chromosomal disorders, many of which lead to serious disability and early death (see also Chapter 1).