Human pregnancy wastage is probably of the order of 50%. It is greatest in the earliest weeks of conception when there is a close correlation with defective embryogenesis, approximating to 100% in the first 3 weeks.
Approximately 36% of all clinically recognized abortuses have a chromosome anomaly. This figure does not represent the primary incidence of heteroploidy at conception, because it excludes pre-implantation and early implanted losses which are not recognized clinically.
This heteroploidy is most common in early pregnancy at the time when pregnancy wastage is highest. It is not a homogeneous problem but involves a wider range of chromosome anomalies than those found in the adult, autosomal trisomies, XO monosomy and triploidy being the most common findings.
It follows that errors in gametogenesis, fertilization and early cleavage divisions are a major cause of prenatal mortality. Biological factors which predispose to these faults include maternal ageing, pre-ovulatory and post-ovulatory ageing of the oocyte and ageing of the spermatozoon. Parental chromosome rearrangements are of minor importance as a cause of chromosome anomalies in abortuses, although they may be of greater significance in recurrent abortions.