But Louise Brinton, chief of hormonal reproductive epidemiology at NCI, cited another possible explanation for the findings. Most of the women taking fertility drugs were taking clomiphene, a first-line treatment for infertility. Clomiphene is in the same drug family as the chemotherapy agent tamoxifen, which might explain why some women taking it appeared to have a reduced risk of breast cancer.
Also, pregnant women, especially those who are age 30 and older, are at an increased risk of breast cancer for a few years after they give birth, whether or not they have taken fertility drugs.
"You would expect to see that women with recent pregnancies would be at increased risk," said Brinton, who was not invovled in NIH study. "Then you start to see that risk turn into lifetime protection relative to women who have never had a pregnancy."
Weinberg said future research on fertility drugs and breast cancer should consider the role that pregnancy plays in affecting younger women's risk for developing the disease.