We used to have a rule of thumb which said that women should wait two years after a diagnosis of breast cancer to try and get pregnant -- there was always some fear that the surge in estrogen that accompanies pregnancy might increase the risk of recurrence.

Using a registry in Western Australia, women with breast cancer were studied from 1982 through 2003. Of the five percent of women who became pregnant after their diagnosis there seemed to be no increased risk for recurrence or change in survival from breast cancer.

Chemotherapy did not affect pregnancy outcome for women who became pregnant at least six months after diagnosis.

The rule of thumb for 2 years may have come from data suggesting the risk of recurrence is highest in the first 2 years and we thought it best to finish chemo (when it is offered) and get two years of tamoxifen in prior to trying to conceive.  Of course tamoxifen should be stopped for several months before trying to conceive as it could harm the fetus.  This is a difficult area to tease out so if you are interested in getting pregnant following your diagnosis please discuss all the issues with your doctor.

Reference:  Ives A, et al. Pregnancy after breast cancer: population based study. British Medical Journal, 2006