Influence of birth order and maternal age on the human sex ratio at birth.

Abstract

Data published by the U.S. Bureau of the Census in the 1940s and the British National Office of Vital Statistics in the 1930s and 1940s has been used to examine the influence of birth order and maternal age on the sex ratio at birth. In view of the substantially different sex ratios of whites and nonwhites the 2 groups have been considered separately. Among the conclusions for white births are: 1) the sex ratio of live births decreases with both increasing birth rank and maternal age; 2) when birth rank if fixed no consistent association with maternal age is seen but the downward trend remains in each maternal age group; and 3) the sex ratio of stillbirths is high at the extremes of birth rank and maternal age. For nonwhite births the statistics show that: 1) when the association between maternal age and birth order is separted the sex ratio of live births shows no consistent relationship to maternal age but does show an increase from birth order 1 to birth order 2 then it declines with increasing birth rank; and 2) stillbirths show a U-shaped trend with both birth rank and maternal age. The base of the U is at higher birth ranks and maternal ages than for whites.