Natural selection and reproductive behavior.

We studied the relaxation of natural selection affecting the newborn population in Italy between 1930 and 1993 due to the decrease in the stillbirth (mortality) rate and the simultaneous changes in women's reproductive behavior (strategies). Results show that, apart from a drastic overall reduction, the stillbirth rate has varied among different groups of neonates. The present stillbirth rate of less than 5 per thousand, observed in 8 of the 20 phenotypic classes defined on the basis of maternal age at delivery and neonate birth order, most likely represents an unavoidable biological or genetic cost. A 9 per thousand stillbirth rate, about twice the potential minimum, exists among neonates born to women delivering at advanced age (> or = 35): the risk they face at the first or, in adverse living conditions, the fourth pregnancy could, however, be decreased by a policy aimed at countering the tendency to delay maternity and improving antenatal care in economically disadvantaged areas of the country.