Background: Previous research assessing the impact of pregnancy and age at first pregnancy on breast cancer risk in BRCA1 and BRCA2 mutation carriers has produced conflicting results, with some studies showing an increased risk following early first pregnancy in contrast to the reduced risk in the general population of women. Methods: Female BRCA1 and BRCA2 carriers from North West England were assessed for breast cancer incidence prior to 50 years of age comparing those with an early first full-term pregnancy (<21 years) to those without a full-term pregnancy. Breast cancer incidence per decade from 20 years and Kaplan-Meier analyses were performed. Results: 2424 female mutation carriers (1278 BRCA1; 1146 BRCA2) developed 990 breast cancers under the age of 50 years. Women who had their first term pregnancy prior to age 21 (n=441) had a lower cancer incidence especially between age 30-39 years. Kaplan-Meier analysis showed an odds ratio of 0.78 for BRCA1 (p=0.005) and 0.73 for BRCA2 (p=0.002). Conclusions: The present study demonstrates a clear protective effect of early first pregnancy on breast cancer risk in both BRCA1 and BRCA2 mutation carriers.