The karyotypes of 941 singleton and 42 twin abortuses and 4 cystic placentae were determined. 30.5% of the singletons were chromosomally abnormal; 49.8% of these were trisomic, 23.7% X-monosomics and 17.4% polyploid. 143 trisomies were identified by banding; over a third had an extra chromosome 16, more than 10% an extra 21 or 22 and about 5% an extra 2, 18 or 15. Examples of trisomy 3, 4, 8, 9, 10, 13, 14 and 20 were also encountered. Using the data from two other published studies, the prevalence of different trisomies was estimated and an attempt was made to relate the karyotype of the conceptus to its subsequent development.

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Summary The effect is described of maternal factors on the proportion of fetal chromosomal anomalies in a series of 2620 spontaneous abortions, of which 992 specimens were karyotyped. Maternal age was the most important factor associated with a rise in the proportion abnormal, followed by Social Class I or II and the use of oral contraception before conception. The problem of extrapolating from the proportion abnormal to absolute incidence of anomalies is discussed. In the case of increasing maternal age, the evidence suggests that the rise in incidence of spontaneous abortions with age is accounted for by an increased incidence of chromosomally abnormal fetuses. In the case of high social class and a history of oral contraception, the evidence on incidence is scanty and the rise in the proportion abnormal may either reflect a decrease in the abortion rate of chromosomally normal fetuses, or a small increase in the incidence of lethal chromosomal anomalies.

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