“We have an ounce of prevention that could save lives,” said Dr. Bhuvaneswari Ramaswamy, the paper’s senior author and an associate professor of medical oncology at Ohio State University in Columbus. “But are we fully educating the mothers when they make this difficult choice? Because it is not an easy choice.”

While companies market infant formula by claiming their products are effective substitutes for breast milk, Dr. Ramaswamy said, “formula is not going to help women live longer and be there for their families.”

The new study surveyed 724 women aged 18 to 50 who had given birth to at least one child. The vast majority of them had breast-fed.

Just over half knew before they gave birth that breast-feeding reduced the risk of breast cancer, and over a third of those said the information influenced their decision to breast-feed.

But only 120 of the women said that their health care providers had informed them about the implications for their own long-term health. Most of those who knew about the health advantages to nursing moms had gleaned the information from popular media or the internet. And these women tended to breast-feed for much longer — 13 months on average — than women who did not know about the health implications, who breast-fed for only nine months on average.