Objective: The increased risk of breast cancer in women with family history of breast cancer (FHBC) might be reduced by early childbirths. However, childbirth induces a transient increase in risk in the first 5-10 years, which coincides with the relatively increased risk of family cases at a young age. The objective was to investigate this short-term change in risk according to FHBC. Methods: We used a population-based cohort of 1.5 million Danish women. Between 1968 and 1990, 2770 incident cases of breast cancer below 40 years of age were identified in the Danish Cancer Register, of whom 276 (10%) had a FHBC. Results: The first 5 years after a birth the short-term increase in risk was 30% (3-64%) larger in women with FHBC than without FHBC. After the first 5 years we observed no difference in the effect of a birth between women with and without FHBC. Conclusions: The adverse short-term effect of childbirth is stronger in women with FHBC.