The sex ratio of human births related to maternal age.

Data from the Annual Reports of the Registrar-General for England an d Wales (1939-1947) and for Scotland (1939-1947) were examined to determine the relationship of sex ratio to maternal age. Sex ratios of total births and live births decrease with maternal age; sex ratios of stillbirths increase. The increase in the sex ratio of stillbirths with maternal age accounts for the difference between live births and total births. The decrease in the sex ratio of total births with maternal age may be explained by changes in the sex ratios of abortions earlier than the 28th week. A trend in the sex ratio at conception would then be absent with regard to maternal age. Data from the Annual Reports of the Registrar-General for Scotland (1939-1946), which give the causes of stillbirth were used to investigate the increase of the sex ratio with maternal age. There were 4 main classes of stillbirth described and each class has a different sex ratio. Changes in the sex ratio of all stillbirths with maternal age were in part attributable to changes in the relative proportions of these classes in each age group. The class which exhibits the most definite age trend - anomalies of fetus, placenta, or cord - was examined. Results indicate that each class is influenced by individual causes within the class. Trends of stillbriths with maternal age for any single cause were absent. It is suggested that the reported trend of the sex ratio in association with birth order may also be explained by changes in the composition of stillbirths and abortions with parity.