However, the researchers found that in vitro fertilization, or IVF, patients who first started infertility treatment at age 24 were about 1 1/2 times more likely to eventually develop breast cancer than women who'd received other infertility treatments at the same age.
In vitro fertilization, or IVF, is a procedure in which a woman's egg is fertilized by sperm outside of the body. You may have heard the term "test tube baby" associated with IVF in past years. To help the pregnancy succeed when the fertilized egg is implanted in the woman, extra estrogen is often given, which can cause a temporary but significant rise in the hormone in the body, peaking at more than 10 times the level in a normal menstrual cycle, the researchers write.
And "reproductive hormones, particularly estrogen, have long been implicated in the development of breast cancer," they say.
Previous studies of the relationship between IVF and breast cancer risk, which were smaller and had shorter follow-up than the new one, have had conflicting results.
IVF, Breast Cancer Risk: Studying the Link
In the new study, researchers followed all women aged 20 to 44 who had been hospitalized in Western Australia for diagnosis or treatment of infertility between 1983 and 2002. Their average age at their first visit was 31, and a quarter were under 28, says researcher Louise Stewart, BSc, of the School of Population Health at the University of Western Australia.
About a third of the 21,025 women in the study underwent IVF. These women were followed from their first infertility admission until Aug. 15, 2010, or the date of their breast cancer diagnosis or death, whichever came first. Three-quarters of the breast cancers diagnosed in study participants were in women 50 or older.
Stewart declined to speculate about why her study found an increased risk of breast cancer in IVF patients who first went to an infertility clinic at age 24 but not in the women who started the process in their 30s or 40s.
The study confirmed the known association between age at first delivery and breast cancer risk. Women who have their first child after age 30 have a small increase in breast cancer risk regardless of whether they have undergone IVF.
Although the study results should be "reassuring" to women who begin IVF treatment in their 30s and 40s, "women should be aware that delivering their first child late in reproductive life, whether assisted by IVF or not, is associated with an increased risk of breast cancer," the researchers write.
The study also found that delivery of twins or other multiples was associated with a reduced breast cancer risk, which has also been observed in some previous studies.
The study "provides another piece in the puzzle of risk factors and the likelihood of getting it [breast cancer] or not getting it," says Barbara Collura, MA, executive director of RESOLVE: The National Infertility Organization. "Is it going to make women think differently about IVF or not? I don't think so."
Breast cancer is perhaps the furthest thing from women's minds when their doctor suggests IVF, Collura says. "Your thought is, 'will this get me pregnant'" she says, followed by concerns about paying for it.
Sometimes a possible connection between the drugs used in IVF treatment and breast cancer does come up in hindsight, though, especially in women over 35 who've had more than three cycles, Collura says. "It's this after-the-fact kind of question people have, and they want some data."
Stewart's study doesn't have an answer for those women, however. Half of the women in her study underwent only one or two cycles of IVF, and information about the types or doses of fertility drugs they received weren't available. The researchers suggest a follow-up study of women who undergo a greater range of cycles to see if there's a connection between IVF "dose" and breast cancer rate.
Stewart's study appears online in the journal Fertility and Sterility.