Previous studies have established that higher birthweight is associated with increased risk of breast cancer. However, the mechanisms underlying this association remain unclear. We explored whether maternal pregnancy weight gain and pre-pregnancy body mass index (BMI), which influence birthweight, are associated with risk of breast cancer in offspring. The Nurses' Mothers case-control study of breast cancer was nested in the Nurses' Health Study I and II cohorts. Mothers of 814 nurses with and 1,809 nurses without breast cancer completed questionnaires with information on pre-pregnancy height and weight, pregnancy weight gain, and other aspects of their pregnancies with the nurse daughters. We calculated odds ratios for breast cancer using conditional logistic regression. Mean pregnancy weight gain was 23 lb, and average pre-pregnancy BMI was 21 kg/m². Mothers' weight gain during pregnancy was not associated with the daughters' risk of breast cancer. Compared to women whose mothers gained 20-29 lb, women whose mothers gained less than 10 lb had a relative risk of 0.92 (95% confidence interval [CI]: 0.62-1.36), adjusting for the age of the nurses. Women whose mothers gained 40 or more pounds had a relative risk of 0.82 (95% CI: 0.55-1.23). Mothers' pre-pregnancy BMI was not associated with the daughters' risk of breast cancer. Women whose mothers had a pre-pregnancy BMI of 30 or more had a relative risk of 0.77 (95% CI: 0.34-1.74) compared to those with BMI less than 20. Additional adjustment for prenatal factors or for nurses' characteristics later in life had no effect on the results. The association between birthweight and breast cancer risk is likely due to factors independent of mothers' weight gain during pregnancy or pre-pregnancy BMI. Because BMIs and pregnancy weight gains were lower in this population than today, we cannot rule out associations for very high pre-pregnancy BMIs or pregnancy weight gains.