Women undergoing in vitro fertilization have long worried that the procedure could raise their risk for breast cancer.

After all, the treatment requires temporarily increasing levels of certain sex hormones to five or 10 times the normal. Two of those hormones, estrogen and progesterone, can affect the course of certain kinds of breast cancer.

A series of studies over the past decade suggested that these former patients may have little to worry about. Experts remained cautious, however, because women who had undergone I.V.F. in the 1980s had not yet reached menopause by the time of the research.

But the largest, most comprehensive study to date, published Tuesday, provides further reassurance: It finds no increased risk among women who have undergone I.V.F.