Analyses of 13,863 births occurring on Guam before 1942 reveal statistically significant concordance of maternal and offspring birth months, accounting for over 17% of recorded births. A secular change by maternal birth year in month of highest concordance level coincides with changing public health measures over this historical period. Together with observed birth order effects, showing decreasing concordance values with increasing birth order rank, these findings suggest an immunological component of observed birth seasonality patterns. The potential effects of maternal-fetal immunological interactions merit greater consideration in birth seasonality studies among historical and anthropological populations. © 1995 Wiley-Liss, Inc.