Risk factors for the increasing trend in low birth weight among live births born by vaginal delivery, Brazil

Objective To identify risk factors for low birth weight (LBW) among live births by va ginal delivery and to determine if the disappearance of the association bet ween LBW and socioeconomic factors was due to confounding by cesarean secti on. Methods Data were obtained from two population-based cohorts of singleton live birt hs in Ribeirao Preto, Southeastern Brazil. The first one comprised 4,698 ne wborns from June 1978 to May 1979 and the second included 1,399 infants bor n from May to August 1994. The risks for LEW were tested in a logistic mode l, including the interaction of the year of survey and all independent vari ables under analysis. Results The incidence of LBW among vaginal deliveries increased from 7.8% in 1978-7 9 to 10% in 1994. The risk was higher for: female or preterm infants: newbo rns of non-cohabiting mothers: newborns whose mothers had fewer prenatal vi sits or few years of education; first-born infants: and those who had smoki ng mothers. The interaction of the year of survey with gestational age indi cated that the risk of LEW among preterm infants fell from 17.75 to 8.71 in 15 years. The mean birth weight decreased more significantly among newborn s from qualified families, who also had the highest increase in preterm bir th and non-cohabitation. Conclusions LEW among vaginal deliveries increased mainly due to a rise in the proporti on of preterm births and non-cohabiting mothers. The association between ce sarean section and LEW tended to cover up socioeconomic differences in the likelihood of LEW. When vaginal deliveries were analyzed independently, the se socioeconomic differences come up again.