Reproductive factors and breast cancer risk. Effect of age at diagnosis.

The data from a French case-control study of 495 patients with breast cancer and 542 control subjects interviewed in five French public hospitals, were analyzed to assess the effect of reproductive factors (age at menarche, age at first full-term pregnancy, the time interval between these two ages, and parity) on the risk of breast cancer. Age at menarche, age at first full-term pregnancy, the time interval between these two ages, and parity appeared to have a limited influence on breast cancer risk. However, the relationship between these factors and the risk of breast cancer varied according to the age at breast cancer diagnosis. In the youngest group of women, the most consistent effects came from factors occurring early in life (menarche, first full-term pregnancy, and consequently the time interval between these two events). These factors had a null or weak effect on the oldest group of women. The protective effect of high parity was confined to the oldest group of women.