In-vitro fertilization involves removing eggs from the uterus and placing them in a laboratory dish, where they are fertilized by sperm before being returned to the woman's uterus. Women undergoing IVF are given hormones to boost ovulation, and there has been concern over whether the hormones increase the risk of certain cancers.

The study analyzed data on 29,700 women provided by 10 clinics over the course of 22 years, making this the largest research project of its type ever conducted. Of the study group, 20,656 women were exposed to fertility drugs, and 9,044 were not. The exposed group showed no greater rates of breast or ovarian cancer than was expected of the general population.  Researchers expected to find 155 cases of breast cancer and found 143. They predicted and found 13 cases of invasive ovarian cancer.

Invasive ovarian cancer is the most common form of ovarian cancer. This study did not determine whether fertility drugs affected the women's risk of developing ovarian tumors of low malignant potential. These tumors, also known as borderline tumors, have been linked to fertility drug use in some, but not all, earlier studies. Borderline tumors tend to spread within the pelvis and abdomen. However, they do not spread through the bloodstream as often as invasive ovarian cancers. Although borderline tumors tend to have a better prognosis than invasive cancers, some cases may be fatal.

The study also suggested there is an increased risk of cancer in very small sub-groups of women. Women with unexplained infertility had significantly more cancers of the ovary and uterus than expected, whether or not they had taken fertility drugs. Also, more cases of breast cancer were diagnosed in the first year after treatment with fertility drugs than researchers predicted: nine were predicted and 17 were detected.

Alison Venn, PhD, an epidemiologist at La Trobe University in Melbourne, Australia, and lead author of the study, speculated that the higher number of breast cancer diagnoses within a year after fertility treatment might be due to increased medical supervision for those women at that time rather than an increase in cancer cases. It might also be that hormonal changes as a result of pregnancy or fertility treatment could stimulate the growth of existing tumors, though they wouldn’t cause the cancer. "You can’t cause breast cancer that quickly," Dr. Venn said.

As for why women with unexplained infertility had higher-than-expected rates of ovarian and uterine cancer, Dr. Venn believes, in some of these women, infertility might have been a symptom when the underlying disease was cancer.

Debbie Saslow, PhD, director of breast and cervical cancer for the American Cancer Society (ACS), agreed with that assessment. "The reason for infertility might be the same as what is causing women to develop ovarian cancer," she said. "A great deal depends on how much estrogen a woman has been exposed to. All of this is clearly related to hormones, but more research is needed to understand the mechanisms involved."

Dr. Venn said the women in the study should be followed as they grow older to try to understand the reasons for increased risk of certain cancers in women with unexplained infertility.