Having twins reduces the risk of many types of women's cancer and starting a family later in life increases risk of breast cancer and melanoma but reduces the risk of ovarian, cervix and uterine cancer according to an international study conducted by a team from The Queensland Institute of Medical Research (QIMR).
The purpose of the study, which involved over one million Swedish women, was to determine the effect of hormonal factors on a number of different cancers, including cervical, colorectal, melanoma and thyroid cancers. The women who were in the study had delivered babies between 1961 and 1996 and were recruited from the Swedish civil birth register and the cases of cancer were identified through the Swedish cancer registry.
"Interestingly we found that having more children offered protection against breast, ovarian and endometrial cancers but the older a woman was when she had her first baby increased her risk of breast cancer but reduced the risk of ovarian and endometrial cancers" said QIMR scientist Steven Darlington who was a Chief Investigator on the study.
Overall, giving birth to twins was associated with lower risks of breast, colorectal, ovarian and uterine cancers. The delivery of twins did not increase the risk of any of the cancers studied.
"Studying the relationship between cancer occurrence and unusual features of reproductive history, such as twinning, may help to shed light on how hormones influence cancer development. We know that the hormone levels of women who bear twins differ from women who have single pregnancies," said Mr Darlington.
Of the one million plus women in the study, more than 25,000 had at least one twin pregnancy and 183 women had two or three twin pregnancies.
The study also found that women who had given birth to at least one set of twins had a reduced risk of colorectal cancer and that this protective effect was increased by the more pregnancies that a woman had.
"Interestingly, it seems that hormonal factors do play at least some role in the development of colorectal cancers, and this finding is supported by other studies which show that colorectal cancer is significantly increased in nuns who generally do not have children."
"It is a well known fact that the more children a woman has, the lower her risk of breast, endometrial and ovarian cancer and we also know that the age that a women has her first and last pregnancy is significant in terms of cancer risk."
Further research into the hormonal influences on different cancers will lead to improved therapies and prevention methods in the future.