Human Sex Ratio at Birth and Mother’s Birth Season: Multivariate Analysis

We used a population-based historical French Canadian database to examine the 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 (^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-April, May-July, August-October, NovemberJanuary). Season of child's birth significantly affected the sex ratio (X2 = 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 variation of sex ratio (/2 = 15. 196, d.f. = 3, p = 0.002); mothers born in FebruaryApril 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 (/2 = 0.618, d.f. = 3, p = 0.892). When a multiple logistic model was applied to the data, mother's birth season was the single most significant factor. The lower sex ratio from mothers born in FebruaryApril was observed consistently for every maternal age and delivery season. Seasonal influences on female fetuses seem to have changed their future reproductive characteristics. It has been reported that sex ratio at birth, or the secondary sex ratio, can change according to season, for example, among US births (Huntington 1938; Lyster 1971; Slatis 1953). If a significant proportion of conception is lost in a particular season during pregnancy in a manner that depends on fetal sex, the secondary sex ratio would be affected accordingly. In addition to the season of a child's own birth, the season of the mother's birth has been associated with variation in the sex ratio of her first-born offspring (Miura et 1 Department of Hygiene, Teikyo University School of Medicine, 2-11-1 Kaga, Itabashi-ku, Tokyo 173-8605, Japan. 2 Programme de Recherche en Demographie Historique (PRDH), Department de Demographie, Universite de Montreal, Quebec, Canada. Human Biology, October 1999, v. 71, no. 5, pp. 875-884. Copyright © 1999 Wayne State University Press, Detroit, Michigan 48201-1309