Background: Studies suggest that gestational weight gain (GWG) and breastfeeding behavior may influence long-term maternal abdominal fat mass. However, this could be confounded by abdominal fat mass before pregnancy because it is unknown whether abdominal fat mass, independently of body size, affects GWG and breastfeeding behavior. Objective: We investigated how maternal prepregnancy fat distribution, described by waist circumference (WC) and body mass index (BMI), is associated with GWG and breastfeeding behavior. Design: We analyzed 1371 live births to 1024 women after enrollment in the Coronary Artery Risk Development in Young Adults study (1985-1996). For each birth, maternal prepregnancy BMI and WC were measured at year 0 (baseline), 2, 5, or 7 examinations. Recalled GWG and breastfeeding behavior were collected at years 7 and 10. GWG was analyzed by using linear regression and breastfeeding behavior by using logistic regression and discrete-time logistic regression. Results: Adjusted for potential confounders, a 1-cm larger WC adjusted for BMI was associated with a 0.19-kg (95% CI: 20.29-, 20.10-kg) lower GWG. In contrast, a 1-unit higher BMI adjusted for WC was associated with a 0.27-kg (95% CI: 0.06-, 0.47-kg) higher GWG. The OR for ever breastfeeding compared with never breastfeeding was 0.93 (95% CI: 0.90, 0.97) per 1-cm larger WC after adjustment for BMI, whereas it was 1.10 (95% CI: 1.02, 1.19) per 1-unit higher BMI adjusted for WC. Conclusions: Maternal prepregnancy body size was differently associated with GWG and breastfeeding behavior depending on the location of the fat mass. Thus, maternal fat distribution may be a more important determinant of GWG and breastfeeding behavior than BMI alone.