Background: We tried to explain why a marked decrease in birthweight of 122 g occurred over a 15-year period in Ribeirão Preto, Brazil.
Methods: Factors reflecting biological, social, and health care characteristics (infant gender, parity, maternal age, marital status, type of hospital, maternal smoking, preterm birth, small for gestational age [SGA], and prenatal care) were assessed on 6711 newborns in 1978/1979 and 2838 in 1994 using multiple linear regressions.
Results: The birthweight distribution shifted to the left and the residual distribution of small preterm babies increased from 1.9% to 3.4%. Only marital status and preterm delivery would have decreased the difference in birthweight over time, explaining for each of them around 30 g of the 122 g. Increasing levels of attendance at antenatal care over time might have decreased the birthweight difference by 40 g. Maternal age and SGA explained little of the decreasing trend. Reductions in maternal smoking would have increased mean birthweight slightly. In stratified analysis the downward trend was more marked among mothers with high education (-202 g) and those delivered by caesarean section (-194 g). After adjusting for all those significant variables mean birthweight was still 74 g (95% CI: -97, -50 g) lower in 1994 than in 1978/1979.
Conclusion: The trend could be explained in part by factors related to marital status that might reflect dysfunctional families in the Brazilian context and the preterm increase that might be associated with advances in medical technology. The high attendance at antenatal clinics or factors associated with it might have prevented a further decrease in birthweight. Our results may be compatible with the high economic development of Ribeirão Preto within Brazil, together with factors associated with its unfavourable lifestyle.