Human sex ratio at birth and mother's birth season: Multivariate analysis

We used a population-based historical French Canadian database to examine t he effects of mother's birth season on sex ratio at birth. Non-first births in the database (n = 127,658) were analyzed for their sex, parish size (2 large parishes of Montreal and Quebec or the other smaller parishes), time period (births up to 1719 or those from 1720), maternal age (less than or e qual to 24, 25-29, 30-34, 35+ years), sex of the preceding sibling (male or female), and birth seasons of the child and his or her parents (February-A pril, May-July, August-October, November-January). Season of child's birth significantly affected the sex ratio (chi(2) = 11.507, d.f. = 3, p = 0.009) , with the births in February-April or May-July showing a lower sex ratio. Season of mother's birth also contributed highly significantly to the varia tion of sex ratio (chi(2) = 15.196, d.f = 3, p = 0.002); mothers born in Fe bruary-April had a low sex ratio among their children (sex ratio = 1.013). In contrast, season of father's birth did not affect the sex ratio (chi(2) = 0.618, d.f. = 3, p = 0.892). When a multiple logistic model was applied t o the data, mother's birth season was the single most significant factor. T he lower sex ratio from mothers born in February-April was observed consist ently for every maternal age and delivery season. Seasonal influences on fe male fetuses seem to have changed their future reproductive characteristics .