Secular trends in birth weight in S. Paulo city, Brazil (1976-1998)

Objective Data from two household surveys on infant and child health status undertaken in the mid-80s and mid-90s, complemented with previous data col lected from maternity hospitals records and more recent data provided by th e state system on birth registries allowed to characterize and analyse secu lar trends in birth weight in the city ofS. Paulo, Brazil. Methods The household surveys included random samples of children under 5 y ears old (n=1,016 children in 1984-85 and n=1,280 children in 1995-96). A r andom sample of births that took place in the city's hospitals in the year of 1976 (n=5,734) was drawn from the hospital records. Birth registries ref er to children born in the city between 1993 and 1998 (around 200 000 per y ear). The study of the social distribution of birth weight took into accoun t the per capita family income and maternal schooling. For the analysis of the determinants of secular trends, hierarchical causal models, multivariat e regression analyses and calculations analogous to the ones used to assess population attributable risks were applied. Results/Conclusions Birth weight distribution in S. Paulo city (an average of 3,160 g and 8.9% of the values <2,500 g) is below the expected for optim um fetal growth conditions (average 3,400-3,500 g and 4-5% of the values <2 ,500 g). The birth weight distribution did not change substantially along t he study period (1976-1998). However there are evidences of changes when di fferent socioeconomic strata are considered separately. Among the lower str ata trends have been positive and this seems to be due to increases in intr auterine growth as a result of an improvement in family's purchasing power women's weight and height, prenatal care and, possibly, the reduction in sm oking. Among the higher socioeconomic strata, birth weight trends have been negative apparently due to are increase in premature births of unknown ori gin.