The human sex ratio: effects of maternal age

Previous studies suggest that stressful pregnancies may be associated with a lower proportion of male relative to female offspring. Low or high maternal age may represent stress for the fetus. Our aim was therefore to study whether the sex ratio differs by maternal age in all pregnancies, and in separate analyses, to assess the sex ratio in pregnancies complicated by pre-eclampsia, fetal death, preterm delivery or small for gestational age (SGA) offspring.

Data from all births in Norway from 1967 through 2006, a total of 2 206 040 births, were used to estimate sex ratios (the number of male per 100 female offspring) according to maternal age. The analyses were done among all pregnancies, and within subgroups of complicated pregnancies. In addition, we estimated the odds ratio (OR) of having a male offspring by maternal age in all pregnancies with adjustment for pre-eclampsia, fetal death, preterm delivery or SGA offspring.

Overall, there was no association of maternal age with the human sex ratio. In subgroups of complicated pregnancies (pre-eclampsia, fetal death, preterm delivery and SGA offspring) the sex ratio was increased. However in pregnancies with pre-eclampsia, the proportion of males decreased with increasing maternal age. In multivariable analyses including all pregnancies, with adjustment for complications, there was still no association of maternal age with offspring sex. However, in pregnancies with SGA offspring, the adjusted OR of delivering a boy at term was lower than expected (OR 0.87, 95% confidence interval 0.85-0.89).

The lower proportion of male births at high maternal age in pregnancies with pre-eclampsia and in pregnancies with live born SGA offspring born at term, supports the hypothesis that male fetuses are more vulnerable to maternal stress than female fetuses. The main limitation of our findings is lack of statistical power due to small study subpopulations.