Induced abortion data provided an estimate of 4.7% numerical chromosome anomalies for women with a clinically recognized pregnancy at the end of the 7th week after the last menstrual period (LMP). This frequency requires that 51.9% of spontaneous abortions occurring after the 7th week should be chromosomally abnormal if the frequency of numerical chromosome anomalies at term (live and stillbirths combined) is 0.53%. Cytogenetic data from surveys of spontaneous abortion suggested a lower incidence of 39.7%. However, this figure is likely to be an underestimate because chromosome anomalies are almost certainly over-represented among the many early abortuses which lack embryonic tissue and hence are not karyotyped. The frequency of numerical chromosome anomalies at conception, arising from meiotic errors, was estimated from sperm karyotype data combined with information on the relative frequencies of maternal and paternal errors in the aetiology of trisomies. This provided a minimum estimate of 20%.
PIP: The incidence of numerical chromosome anomalies in chorionic villus specimens from pregnancies aborted by the Karman procedure at 7-8 weeks gestation was determined, and the results used to estimate the incidence of chromosomally abnormal spontaneous abortions after 7 weeks. The 271 specimens were incubated overnight in culture medium to maximize recovery of metaphases, with 96% success. 10 metaphases were counted for each specimen, enough to detect monosomies, trisomies, triploids, tetraploids and most mosaics. The overall estimate was 4.6% numerical anomalies for this sample at 43-56 days gestation. There were 12.9% among women over 35, compared to 3.5% in younger women. Since the frequency of numerical chromosome anomalies is 0.53% for live and stillbirths combined, the incidence of such anomalies in spontaneous abortions after 7 weeks LMP was estimated at 51.9%. This figure was arrived at by adjusting the present data by maternal age, and multiplying the anomaly frequency by the total number of spontaneous abortions for each week using data from published reports. Both of these variables differ in various reports, because of sampling difficulties, which are described and evaluated. It is probable that the frequency of numerical chromosome anomalies at conception in humans is at least 20%, not counting those arising from diaspermy and cleavage errors.