Young women who wish to become pregnant after the treatment of breast cancer are often advised to wait for at least two years before delivering a baby. The reason to wait for a considerable period is that an early pregnancy after breast cancer may influence the recurrence of breast cancer. However, chemotherapy for breast cancer has toxic effects on the ovaries and fertility is shortened for these women. Consequently, the chances of successful pregnancy in these women diminish with time from the diagnosis of breast cancer. Therefore, it is important to define a minimum safe interval after breast cancer diagnosis before pregnancy in order to maximize the chances of a successful pregnancy in young breast cancer patients. Previous studies have failed to reach a consensus about the timing of pregnancy after breast cancer diagnosis. We will perform a population-based study in which we will link various health administrative databases in Ontario. We will compare the risk of death from breast cancer in women who had a baby to those who did not have a baby either before or after the diagnosis of breast cancer. We will determine if delivering a baby before or after breast cancer diagnosis increases the risk of dying from breast cancer. The results from this study will contribute to a rational approach to counseling of young women with breast cancer in regards to their reproductive options.