Despite concerns and some evidence that fertility treatments increase a woman's chances of developing breast cancer, a large, long term study finds no added risk.
Nearly 10,000 women who received drugs to stimulate ovulation were no more likely to develop breast cancer during 30 years of follow-up than those who never used the drugs, researchers report.
Lead author Louise Brinton, chief of the Hormonal and Reproductive Epidemiology Branch at the National Cancer Institute in Bethesda, Maryland, said the findings are reassuring.
Previous studies have raised concerns over the drugs because women are exposed to higher levels of estrogen during fertility treatments. Extended exposure to extra estrogen could increase the risk of breast cancer.
"The other worry is that (these drugs) cause increased ovulation and . . . that could be linked to an increase in breast cancer risk," Brinton told Reuters Health.
For the new study, the researchers analyzed records for 9,892 U.S. women who were followed for some 30 years after having been evaluated for infertility between 1965 and 1988.
About 38 percent of the study participants were exposed to the fertility drug clomiphene and about 10 percent were exposed to drugs known as gonadotropins.
Over the three-decades of follow-up, 749 breast cancers were diagnosed among the study participants.
Overall, women who were exposed to either type of fertility drug were no more likely to develop breast cancer than those who didn't use the medicines to stimulate ovulation.
The researchers did find an increased risk of breast cancer among a small subset of women who were prescribed the highest doses of clomiphene, however.
Current practice limits treatment with the drug to just three to six cycles. But women who were exposed to 12 or more cycles of clomiphene treatment had a 70 percent higher risk of developing breast cancer compared to women not exposed.
Also, women who took gonadotrophins, usually in combination with clomiphene, but were still never able to become pregnant were about twice as likely to develop breast cancer.
Some of that increased risk may be the result of whatever underlying problem caused those women's persistent infertility, the researchers speculate.
For the small group of women who took clomiphene for more than a year, the reasons for the increased risk are less clear, the authors conclude in their report, published in Cancer Epidemiology, Biomarkers & Prevention.
"It's reassuring that if women desire pregnancy and unfortunately have infertility that they can undergo treatment without modification of their overall risk for cancer later," Dr. Kurt Barnhart, president of the Society for Reproductive Endocrinology and Infertility, told Reuters Health.
Barnhart, who was not involved in the new study, is based at the Perelman School of Medicine at the University of Pennsylvania in Philadelphia.
It's important to note that the new study shouldn't change how women are screened for cancer in any way, he added.
The researchers also caution that continued study of women who receive fertility treatments is needed, because many women included in the current study were still fairly young (in their early 50s) by the end of the follow-up, so they had not reached the age range when breast cancer diagnoses are most common.
"We'll be looking at other results as they emerge and keeping our eye on IVF (in vitro fertilization), which is the more common treatment nowadays," Brinton said.