A study published in the medical journal Twin Research and Human Genetics has found that women who have children have a reduced risk of developing certain types of cancer, and that the risk is further reduced with each pregnancy.

Researchers from the Queensland Institute of Medical Research in Australia studied medical data for more than 1.2 million Swedish women who had given birth between 1961 and 1996. Women who had more children had a lower risk of developing breast, uterine, ovarian, and colorectal cancers than women who had fewer children.

The authors noted that it appeared that "an increase in the hormones produced during pregnancy are protecting against cancer," but scientists have not figured out why. The study found that women who had children earlier in life had a lower rate of breast cancer than women who delayed pregnancy.

Other studies have also found that pregnancy and childbirth offer women protection against certain cancers. The authors of the new study said that doctors should be aware of the cancer risks so they can provide more frequent cancer screening for women with fewer or no children.

The study was initially designed to study cancer risks in women who had twins, since they are exposed to different hormone levels than women who have single pregnancies. While the researchers did find lower cancer rates among mothers of twins, it was not enough to be considered significant, they said.

Neale RE, Darlington S, Murphy MF, Silcocks PB, Purdie DM, Talback M. The effects of twins, parity and age at first birth on cancer risk in Swedish women. Twin Res Hum Genet. 2005 Apr;8(2):156-62.