OBJECTIVE: To evaluate factors associated to the increase in insufficient birth weight (IBW) rates in two population-based-cohorts of singletons born alive in Ribeirão Preto, São Paulo, Brazil, separated by 15 years (1978/79 and 1994). METHODS: The first cohort (1978/79) comprised 6,223 newborns and the second (1994), 2,522, excluding low birth weight newborns. Independent variables association with IBW (maternal age, work, schooling and smoking; previous abortion, previous stillbirth, number of live births, maternal marital status, family income, occupation group, type of hospital, mode of insurance, number of antenatal visits, type of delivery; newborn gender and gestational age) were analyzed. The Odds Ratio (OR) was determined using a logistic regression model and 95% confidence limits were calculated. RESULTS: In 1978/79, maternal age <20 years (p=0.014), family income <5 minimum Brazilian wages (p=0.030), <4 antenatal visits (p=0.003), vaginal delivery (p<0.001), female newborn gender (p<0.001), maternal smoking (p<0.001) and preterm birth (p<0.001) were associated with IBW. In 1994, maternal work outside home (p=0.020), female newborn gender (p<0.001), maternal smoking (p<0.001) and preterm births (p<0.001) were associated with IBW. CONCLUSIONS: Socioeconomics variables associated with IBW in 1978/79 were not determinants in 1994. Newborn gender, maternal smoking and preterm birth remained significant determinants of IBW in both moments. Considering the decrease in rate of maternal smoking and the maintenance of female births, the increase in IBW could be explained by increasing rates of preterm birth in this interval of 15 years.
insufficient birth weight; infant, newborn; birth weight; infant, premature
