Secular trends in uncertain-sex births and proportion of male births in Norway, 1967-1998

It has been hypothesized that environmental factors influence sex differentiation in the fetus, thus causing a reduced sex ratio (male/female) at birth, an increase in the prevalence of ambiguous-sex infants, and possibly an increase in spontaneous abortion rates. In Norway, subsequent to 1967, all deliveries, including late abortions after 16 wk of gestation, have been reported to the Medical Birth Registry of Norway. The authors used these data to assess birth-weight-specific secular trends of uncertain-sex infants and the proportion of male births, from 1967 through 1998. Total rates of uncertain-sex infants increased from 2.3/10,000 births in 1967 to 8.0/10,000 births in 1998 in the birth weight strata of less than 600 gm. The total proportion of male births was stable; however, there was a reduction in the proportion of male births among births weighing less than 400 gm-from 58.4% to 53.4%. Incomplete ascertainment of abortions may have biased the results, but it would not have accounted for the increased occurrence of uncertain-sex infants and the decreased proportion of males observed among late-term abortions.