Dr. Valerie Grant has a website, Sexratio.com, which is a compilation & report of of her research:

This site discusses the theory that the sex of a baby is determined, not solely by chance, but by the nature of the mother. There may be a natural selection process that alters the probability of a mother having a boy or girl, depending on some of her characteristics. Some people think this sounds bizarre. But there is plenty of evidence to suggest that the determination of the sex of the infant is not entirely random. Something else besides chance has an influence: the mother's level of testosterone.

This sort of one-parameter-to-rule-them-all obviously needs to be considered in light of other contextual factors. Nevertheless, it is an area where empirical investigation seems likely to yield some findings of note. Finally, note that these sort of hormonally oriented explanations have a strong facultative angle, sex determination bias is not something that is necessarily ingrained at birth because of genotypic considerations, environmental and developmental inputs can strongly modulate the factors.

Related: Male brain ~ more sons vs. female brain ~ more daughters?.