The period before the first birth is traditionally viewed as particularly critical for a woman's breast cancer risk. Nonetheless, the importance of early timing of a woman's first compared with subsequent births is not well understood. In the present study we examine this question using a population-based cohort of 1.5 million Danish women born between 1935 and 1978. Between 1968 and 1994, 13,049 incident cases of breast cancer were identified in the Danish Cancer Registry. According to our results, a woman's breast cancer risk is related to her age at any of her births. The risk increase per 5 year's increase in maternal age at first, second, third, and fourth birth was 9%, 7%, 5%, and 14%, respectively. For fifth and sixth births it was 5%. We observed a risk reduction after any birth occurring before 30 years of age (in uniparous women before 25 years of age). These effects were strongest more than 10 years after birth. Thus, our study shows that early timing of any additional birth induces an additional long-term reduction in maternal risk of breast cancer; that is, early reproductive years, rather than just the nulliparous years, constitute the critical period.