Approximately 60% to 70% of pregnancy losses whether as an isolated event (one) or recurrent (two or more)are numerically chromosomally abnormal. The most common chromosomal abnormality is Trisomy 16 which occurs in 26% of losses. Trisomy 16 is incompatible with life and has never been described progressing more than 11 weeks of gestation. The second most common chromosome abnormality in losses is monosmy X (45,X). It is estimated that 90% or 45,X conceptions spontaneously abort and 10% may go on to term delivery giving rise to little girls born with Turner’s Syndrome. The third most common chromosome abnormality in miscarriages is Trisomy 22 and the fourth is Trisomy 21. It is estimated that approximately 80% of Trisomy 21 pregnancies end in a miscarriage (pregnancy loss before 20 weeks of gestation) or intrauterine fetal demise (pregnancy loss after 20 weeks of gestation), while 20% may progress to term delivery.
The only noncontroversial parental cause of pregnancy loss is a known chromosome abnormality in the couple experiencing loss. Only 3% of individuals having three or more pregnancy losses have an inborn chromosome abnormality themselves which does not explain the over 60% of chromosomally abnormal losses. Most chromosome abnormalities in early pregnancy losses arise spontaneously either through non-disjunction or other genetic errors in gametes (egg or less likely sperm) or the early embryo.