Does age at the last birth affect breast cancer risk?

We studied the relative importance of age at first and age at last full-term pregnancy (FTP) in a large data set of 3950 cases and 11,510 controls, of whom 2263 cases and 7359 controls had two or more FTPs. We found that a 5-year delay in age at first FTP is associated with an odds ratio of 1.17 (95% confidence interval of 1.05-1.29), whereas a 5-year delay in the age of last FTP is associated with an odds ratio of 1.05 (95% confidence interval 0.96-1.14). Misleading results concerning the relative importance of age at first and last FTP may be obtained when parity is only adjusted in broad categories, uniparous women whose FTP is, simultaneously, both the first and the last are not excluded from the analysis, and age at any FTP between the first and last is not controlled for.