Four factors influencing birth weight.

The birth weight of newborn infants is usually studied as a concomitant variable to prematurity or infant mortality and less frequently as the variable of primary interest. This investigation concentrates on the relationship of infant birth weight to four other variables. The combined effect of birth order and mother's age on birth weight is analysed in a large sample of births. The influence of maternal age on the birth weight is investigated for each birth order group and, conversely, the influence of birth order is studied at different maternal ages. Both secular and seasonal trends in mean birth weight, which in general are neglected in the literature, are included here.