Scientists at the Queensland Institute of Medical Research (QIMR) found increasing numbers of pregnancies were associated with a significantly reduced risk of certain cancers.
"The more children you have, the more protective it gets," said medical health statistician Steven Darlington.
"It seems that an increase in the hormones produced during pregnancy are protecting against cancer, but we're not quite sure exactly how or why that happens."
He studied more than 1.2 million Swedish women, including about 25,000 who had delivered twins, to determine the effect of reproductive history on a number of different cancers.
The women had all given birth between 1961 and 1996.
Mr Darlington said Swedish, rather than Australian women, were studied because of Sweden's highly-detailed civil birth and cancer registries.
The study was originally designed to test whether giving birth to twins protected women against types of cancers believed to be affected by hormones.
Women who deliver twins are subject to different hormone levels than those with single pregnancies.
Although the study found twin births were associated with a decrease in the risk of breast, colorectal, ovarian and uterine cancers, this was not statistically significant.
"Having twins is protective but not as significant as having more children," Mr Darlington said.
He said colorectal cancer seemed to be particularly related to hormonal influences.
A study in the 1960s found nuns had extremely high levels of colorectal cancer compared to other women.
"Since then a lot of other studies have been done and there's a great amount of evidence now that an increase in children is a significantly protective factor against this cancer," Mr Darlington said.
The study, published in Twin Research and Human Genetics, also found women who started a family later in life were at greater risk of breast cancer than other mothers.
Mr Darlington, who now works for Queensland Health's breast cancer screening program, said such studies helped scientists better identify women at increased susceptibility of developing certain cancers.
"If you know that women who have less children are more likely to contract breast cancer then you can screen them more frequently," he said.
Nevertheless, he admitted the research had some limitations including an inability to control for factors like miscarriage, abortion, use of the oral contraceptive pill, assisted reproductive technologies, diet and exercise.
By Janelle Miles
May 29, 2005
Original Article at news.com.au