Natural Selection, Selection Bias and the Prevalence of Down's Syndrome

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Abstract
A newborn baby has survived powerful, natural selective forces. For example, only 75 per cent of four-week pregnancies conclude with a live birth.1 From chromosomal analyses of spontaneous abortion products, we know that in early pregnancy there are powerful selective forces against chromosomal anomalies. Fetal chromosomal diagnosis has given us a new view to the selective forces. Hook, in this issue of the Journal, reports that during the last half of pregnancy, selection continues against trisomy 21 and probably trisomies 18 and 13. The higher mid-trimester prevalence of fetal chromosomal disorders for women 35 years of age and older, . . .