Marriage season, promptness of successful pregnancy and first-born sex ratio in a historical natural fertility population - evidence for sex-dependent early pregnancy loss?

We investigated population-based vital records of the seventeenth and eight eenth century French Canadian population to assess the effects of marriage season on the outcome of the first births under natural fertility condition s (n=21,698 marriages). Promptness of the first successful conception after marriage differed according to marriage season; the proportion of marriage s with a marriage-first birth interval of 8.0-10.0 months was lowest (34%) for marriages in August-October (P=0.001). Although the male/female sex rat io of the babies born with an interval of 8.0-10.0 months was generally hig her (1.10) than those with an interval of 10.0-24.0 months (1.05), the marr iages in August-October resulted in a significantly reduced sex ratio (0.96 ) among only the prompt conceptions (P=0.026). We discuss whether this seas onal reduction of the sex ratio could be partly explained by a clustered pr egnancy loss of male zygotes in early pregnancy.