Estimating the risk of breast cancer in relation to the interval since last term pregnancy.

We review the analytical methods of studies that asked whether a term pregnancy transiently increases a woman's risk of breast cancer. These analyses must separate the possible influence of a recent pregnancy from that of two other correlated variables, attained age and age at last pregnancy. Most analyses have compared women of the same parity. To determine, however, whether the risk of breast cancer is different than it would have been had the last pregnancy never occurred, it is necessary to compare the risk of breast cancer over time among women of parity N+1 with women of parity N, which has been done in a few studies. To generate relative risk estimates that are independent of arbitrary coding decisions, we show that the analytical models must be more complex than those in published studies. We used these models to compare women of parity N and N+1 for breast cancer occurrence, using data from the U.S. Cancer and Steroid Hormone Study. For the first 6 years after delivery, first term pregnancy was associated with lower or unchanged risk of breast cancer, second pregnancy with higher risk, third pregnancy with lower risk, and fourth pregnancy with both lower and higher risk depending on age at delivery. Given the inconsistent findings between adjoining levels of parity and the wide confidence intervals around the estimates, we could not find clear evidence for or against the theory that term pregnancy is transiently associated with an increased risk of breast cancer.