In vitro fertilization (IVF) is widely used to help individuals or couples start or grow their family. Part of the process involves stimulating a woman’s ovaries with hormones that have been connected to breast cancer risk, raising concern among doctors and patients. The results of the most extensive study to date on IVF and breast cancer risk may help allay some of these fears.

For the new study, published July 19, 2016, in the Journal of the American Medical Association, researchers reviewed medical records of and questionnaires completed by nearly 20,000 women in the Netherlands who had one or more IVF cycles between 1983 and 1995. As of 2013, the women who had undergone IVF did not have a significantly greater risk of receiving a breast cancer diagnosis than a similar group of 6,000 women who had visited fertility clinics but had not undergone IVF. Women who had seven or more cycles of IVF and those who had not had children before starting IVF had the lowest risk.

The relationship between breast cancer risk and IVF was important to study, says Alexandra van den Belt-Dusebout, an epidemiology researcher at the Netherlands Cancer Institute in Amsterdam, because “with the large numbers of women undergoing ovarian stimulation for IVF, even a small risk increase would have important public health implications.”

As women age, overall breast cancer risk increases. The average age of the women who entered the study was 32. Currently only 14 percent of the women in the study have turned 60. The researchers will continue to follow all of the women as they age, tracking breast cancer diagnoses.

Louise Brinton, an epidemiologist at the National Cancer Institute in Rockville, Maryland, says that, because most of the women in the study are relatively young, it is still too early to draw definitive conclusions about IVF and breast cancer risk. She also notes that there are some factors that may affect fertility, such as obesity, “that can affect your cancer risk, independent of whether you’ve gotten infertility treatment.” Even so, she says, “the results certainly add to other reassuring results” for women who’ve had IVF.