Social determinants of birthweight, ponderal index and gestational age in Sweden in the 1920s and the 1980s.

This study compared the effect of social class and marital status on birth outcomes in Sweden, using (i) data on all births at the Akademiska Hospital in Uppsala from 1920 to 1924 with socioeconomic information from records at birth; and (ii) a linkage of the Medical Births Registry for all births in Sweden in November/December 1985 to the 1985 Census. Preterm births (<37 weeks) have become less common during the 20th century. Between 1920-24 and 1985, mean and median birthweight increased, as did mean ponderal index, indicating a shift to the right of the birthweight and ponderal index distributions. In 1920-24, birthweight and ponderal index were associated with the social class of the household and with the marital status of the mother. Babies of single mothers were lighter and thinner, and had a much greater probability of being born preterm. In contrast, in 1985, maternal marital status (and cohabitation status) had a weaker effect on birthweight and ponderal index. The importance of household social class for ponderal index and preterm birth changed similarly, but its importance for birthweight remained. The mediating mechanism may have changed. Mothers from farming households now gave birth to the heaviest babies (nearly 200 g heavier than those of unskilled workers). Adjustment for a number of factors, including smoking, had a limited effect on these social class differences. In conclusion, biological processes during the foetal period are systematically linked to the social circumstances of the mother, but in a different way in the 1920s and in 1985.