Seasonality of births and family characteristics in a Spanish population.

The seasonality of births in the period 1871-1977 is studied in a rural north-western Spanish population. Based on a total sample of 11,695 birth registrations, temporal variation is analysed. For siblings, according to family reconstitution, the total family size, the legitimacy of the child, and birth order are considered. A coefficient of birth month dispersion is defined and estimated for each family. Intra-family variation is related to inter-family coefficients in order to determine whether the local seasonal pattern of births may be partly explained by family characteristics.