Many countries have experienced a pronounced shift towards later childbearing in recent years. Having children later in life has become a characteristic of contemporary fertility in affluent societies. Childbearing, particularly at a younger age, has been shown consistently to reduce the risk of breast cancer in the long term. Breast cancer constitutes the most common cancer in women and is an important public health concern worldwide. Early age at full-term pregnancy lowers the risk of breast cancer in women, whereas in older women first full-term pregnancy appears to increase the risk. Lactation seems to be relatively important in reducing the risk and abortion is associated neither with increased nor reduced risk. Several hypotheses have been proposed to account for the protective effects of parity. These include an altered systemic hormonal environment and an altered mammary cell fate.