This study examines the differences in the pattern of weight gain according to trimesters of pregnancy for women who delivered term vs. preterm and analyzes the independent effect of prepregnancy weight status and rate of weight gain on delivering preterm. The differential effects of these variables on the etiological pathways of prematurity (preterm labor and preterm rupture of the amniotic membranes) were also examined. Data were collected prospectively from 7589 pregnant women receiving care in public health clinics in the West Los Angeles area. Eighty percent of women identified themselves as being of Hispanic origin. Multivariate logistic regression techniques were used to isolate the role of each nutritional variable from other factors that may influence birth outcome. Women who delivered preterm had patterns of weight gain similar to women delivering term infants. Underweight status (body mass index < 19.8 kg/m2) before pregnancy nearly doubled the likelihood of delivering preterm [adjusted odds ratio (AOR) 1.98, 95% confidence interval (CI) = 1.33, 2.98). Inadequate weight gain in the third trimester defined as < 0.34, 0.35, 0.30 and 0.30 kg/wk for underweight, normal weight, overweight and obese women, respectively, increased the risk by a similar magnitude (AOR 1.91, 95% CI = 1.40, 2.61). Slight differentiation of these risk factors occurred when analyzing the etiological pathways of preterm birth. Preconceptional nutrition counseling and promotion of adequate weight gain during the third trimester of pregnancy should be components of public health programs designed to decrease the prevalence of preterm birth.