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.
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 breastcancer 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.