OBJECTIVE: To examine the association of childbearing with weight and waist circumference (WC) changes, we compared women with and without pregnancies or births during follow-up. STUDY DESIGN: A multicenter, longitudinal observational study over 10 years. Comparison groups defined by the number of pregnancies and births during follow-up: P0 (0 pregnancies; nongravid), P1 (1 + miscarriages or abortions; 'short' pregnancies), B1 (1 birth), and B2 (2 + births). Mean changes in weight and WC for P1, B1 and B2 groups vs P0 were examined separately by race (black and white), baseline parity (nulliparous and parous) and baseline weight status (normal weight; BMI <25 kg/m2 and overweight; BMI > 25 kg/m2). SUBJECTS: A population-based sample of 2070 women aged 18-30 y at baseline (1053 black subjects and 1017 white subjects) from Birmingham, Alabama, Chicago, Illinois, Minneapolis, Minnesota, and Oakland, California were examined five times between 1985-1986 and 1995-1996. MEASURMENTS: Weight and WC measurements were obtained using standardized protocol at baseline and examinations at years 2, 5, 7 and 10. Sociodemographic, reproductive, and behavioral attributes were assessed at baseline and follow-up examinations. RESULTS: Gains in weight and WC associated with pregnancy and childbearing varied by race (P<0.001), baseline parity (P<0.05) and overweight status (P<0.001). Among overweight nulliparas, excess gains in weight (black subjects: 3-5 kg, white subjects: 5-6 kg) and WC (black subjects: 3-4 cm, white subjects: 5-6 cm) were associated with 'short' pregnancies and one or more birth(s) during follow-up compared to no pregnancies (P<0.01 and 0.001). Among normal weight nulliparas, excess gains in weight (about 1 kg) and WC (2-3 cm) were associated with follow-up birth(s) (P<0.05). Among women parous at baseline, no excess weight gains were found, but excess WC gains (2-4 cm) were associated with follow-up births. CONCLUSION: Substantial excess weight gain is associated with both short pregnancies and a first birth in women overweight prior to initiation of childbearing. Excess weight gain was not associated with higher order births. Increases in waist girth were cumulative with both first and higher order births among overweight as well as normal weight women. Interventions to prevent obesity should be targeted at women who are overweight prior to initiation of childbearing. The impact of excess WC gains associated with childbearing on women's future health risk should be evaluated further.