Parental age, parity and sex ratio in births in England and Wales, 1968-77

Variations in sex ratio with maternal age, paternal age, parity, and time are examined in data on all legitimate births in England and Wales, 1968-1977. Significant linear declines in sex ratio with both increasing parity and increasing paternal age, and a curvilinear relationship with maternal age are found. Results show that 1) before 1955 it seems likely that the sex ratio declined with an increase in maternal age, 2) between 1955 and 1968 there was an unexplained decline in the sex ratio of births to women aged 20-24 and an increase in that of births to women aged 30-34, and 3) there was no significant secular trend in sex ratio during the period 1968-1977. Using data provided by the Registrar General of England and Wales, a weighted multiple regression analysis is applied. Some possible explanations for the changes in sex ratios of births during the years studied are: 1) racial heterogeneity, 2) artificial insemination, 3) the pill, and 4) ovulation induction. Overall, some artifact or combination of artifacts has transformed the regression of sex ratio on maternal age from a monotonic decline to a cubic, or this hypothesis stands in need of modification.