Preimplantation genetic screening
© Cambridge University Press 2013. It is well established that chromosome abnormality (aneuploidy) is common during the early stages of pregnancy in humans. A high prevalence of aneuploidy is evidenced by the frequent detection of cytogenetically abnormal gestations following prenatal testing using chorionic villus sampling (CVS) or amniocentesis. Abnormalities affecting chromosomes 13, 18, 21 and the sex chromosomes (X and Y) are particularly common and show increasing prevalence with advancing maternal age. Although aneuploid conceptions can occasionally result in a live birth (e.g. trisomy 21, Down syndrome), the vast majority are incompatible with development to term. In addition to the chromosomes mentioned above, trisomies (three copies instead of the usual two) affecting chromosomes 15, 16 and 22 are frequently observed in tissue from miscarriages. It is estimated that more than two-thirds of all first trimester miscarriages are a consequence of aneuploidy, underscoring the lethality of chromosome imbalance [12, 15] . While it has long been clear that incorrect numbers of chromosomes are relatively common during early pregnancy, molecular cytogenetic data accumulated over the last two decades has revealed that abnormality rates are higher still at the very earliest stages of development. During the 1990s, multiple studies using fluorescent in situ hybridization (FISH) revealed that the majority of human preimplantation embryos contain aneuploid cells (e.g. 3, 16). Analysis of oocytes and/or their corresponding polar bodies yielded similar data, indicating a maternal origin for most of the embryonic abnormalities . Recently, more powerful cytogenetic methods (e.g. microarrays, discussed in greater detail below) have confirmed much of the FISH data, demonstrating aneuploidy rates of over 25% for the oocytes of women below 35 years of age and rates in excess of 75% for women over 40  .