According to a study published in the July 6 edition of the Journal of The National Cancer Institute, the risk of breast cancer in women who failed to conceive at least a 10-week pregnancy and were using fertility drugs was statistically considerably lower than in those who did not use fertility drugs. Although the risk for those on fertility drugs who did conceive a 10-plus week pregnancy was higher than that of unsuccessfully treated women.

Fertility drugs stimulate ovulation in women by temporarily raising their estrogen levels. However, estrogen is known to have a significant impact on breast cancer and whilst some research has found that infertility treatments lead to a higher breast cancer risk, other studies remained inconclusive.

Chunyuan Fei, Ph.D., from the National Institute of Environmental Health Sciences (NIEHS), and his team decided to establish the risk of early-onset breast cancer after using ovulation-stimulating fertility drugs. In a sister-matched case-control study, called the Two Sister Study, the researchers assessed women under the age of 50 with breast cancer and their breast cancer-free control sisters between September 2008 and December 2010. They particularly focused on fertility-drug exposure, which either did or did not result in a pregnancy for a duration of at least 10 weeks.

The finding revealed that women on fertility drugs had a non-statistically considerably lower breast cancer risk than women who took no fertility drugs, whilst those on fertility drugs who failed to conceive a 10-plus week pregnancy had a statistically considerably lower breast cancer risk compared to nonusers. Although, women on fertility treatment who conceived a 10-plus week pregnancy did have a statistically considerably higher breast cancer risk than those who were treated unsuccessfully.

The researchers state: Our data suggest that exposure to a stimulated pregnancy is enough to undo the reduction in risk associated with a history of exposure to ovulation-stimulating drugs.”

According to the researchers, fertility drugs potentially elevate the risk by a change in pregnancy-related breast-tissue remodeling, although the risk of breast cancer in successfully treated women was comparable to that of non-users.

The researchers are aware that their study had some limitations, for example, the reliance on self-reported fertility drug usage as well as lack of data on specific diagnosis for infertility.

According to a linked editorial by Louise A. Brinton, Ph.D., of the Division of Cancer Epidemiology and Genetics at the National Cancer Institute, the study findings are difficult to understand in the context of earlier research regarding the contradicting results from a lowered risk to a higher risk to no relationship between the drugs and the risk of early onset of breast cancer. She explains that one possible explanation could be that the reduced overall risk linked to drug usage may be due to the fact that one of the drugs, i.e. clomiphene, is a selective estrogen receptor modulator (SERM) similar to tamoxifen, an established chemo-preventative.

However, the elevated risk in successfully treated women could be linked to the higher exposure to ovarian hormones, as well as “the dual effect of pregnancy on breast cancer risk, namely a short-term transient increase that dissipates with time and eventually leads to a long-term risk reduction.”

She continues writing that another complicating factor in interpreting the study’s results is the study’s focus on women who developed breast cancer before they reached the age of 50, as this is more frequently linked to genetic factors compared with breast cancers that are diagnosed at a later age.

Brinton concludes writing that further research is needed in order to clarify these links. She states:

“Because of such complexities, results from individual investigations must be cautiously interpreted and weighed against the considerable benefits associated with fertility drug usage, including a high probability of carrying pregnancies to term, which can lead to substantial long-term reductions in breast cancer risks.”

Written by Petra Rattue