One of the earliest clues that having children may help to protect women against breast cancer came from historical studies of nuns.

More than 300 years ago, researchers observed that women who dedicated their life to religious service — most of whom never gave birth — had "some of the highest rates of breast cancer", medical oncologist Sherene Loi said.

"This led scientists to suspect that child rearing and breastfeeding were protective, while being nulliparous [never given birth] was a risk factor," Professor Loi, a clinician scientist at the Peter MacCallum Cancer Centre, said.

Over time, evidence of this protective effect was confirmed, though the reasons behind it were not entirely clear, and thought to be largely the result of pregnancy-related hormonal shifts.

But new research,

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