Association of breast cancer risk with age at first and subsequent births: a study in the population of the Estonian Republic.

Information on reproductive history was obtained from 362 urban breast cancer patients attending the oncological dispensaries at Tallinn and Tartu, Estonian Republic, and from 694 urban women participating in gynecologic screening programs offered by the same centers. The 2 groups were compared by means of Mantel-Haenszel and logistic regression procedures to estimate age-adjusted odds ratios. Women whose first birth occurred before 20 years of age had a breast cancer risk less than one-third the risk of nulliparous women. Risk increased with increase in age at first birth (AFB) but remained below 1.0 (relative to nulliparae), even in the highest AFB categories. The fertility rate in Estonia during the period of this study was relatively low, which facilitated an evaluation of the effect of births subsequent to the first. After adjustment for AFB, it appeared that in this population subsequent births had a protective effect additional to that conferred by the first birth. Moreover, for women who had only 2 children, the age at the time of birth of the second child was a determinant of that effect. Compared to nulliparous women, the breast cancer odds ratio for uniparous women who had their child before age 25 was 0.62, and the ratio for duoparous women who had both their children under that age was 0.18. Neither lactation nor menarche was a risk factor for breast cancer in this pouplation.