Some risk factors for low birth weight among live births from vaginal delivery were studied in two population-based cohorts of singleton live births from families residing in the municipality of Ribeirao Preto, Southeast Brazil. The first comprised all births from June 1978 to May 1979 (4700 deliveries) and the second was a four-month survey, including infants born from May to August (1399 deliveries). Since c-section was the factor more closely associated with the risk of LBW in the city in 1994 and LBW rate has also risen among births from vaginal deliveries we conducted this separated analysis, including only vaginal deliveries. Comparing 1978/79 with 1994 survey, the incidence of LBW among vaginal deliveries increaseed from 7.8% to 10%. The risks for LBW in the 1978/79 cohort were higher for female infants, for those from older mothers, for pre-term infants, for those whose mothers had fewer prenatal visits and for infants of smoking mothers. In 1994, the factors associated with LBW among vaginal deliveries after controlling for confounding were pre-term birth, fewer prenatal visits and non-cohabiting women. This increase in LBW rate has occurred despite the fact that the majority of known risk factors for LBW had a favorable evolution, except for an increase in the percentage of young and non-cohabiting mothers. The highest decrease in mean birth weight was observed among newborns from more qualified families whose families also had the highest increase in the proportion of pre-term birth and non-cohabiting women. It remains unclear whether this upward trend in LBW rate is confined to Ribeirao Preto or if it has also occurred in other Brazilian cities. The role of other non-studied factors in this rise needs to be better investigated.