Every pregnancy potentially reduces a woman's risk of developing endometrial cancer, new research has found.
The Queensland Institute of Medical Research (QIMR) Berghofer found both miscarriages and full-term pregnancies can reduce the risk.
Published in the International Journal of Cancer, the study found that each additional full-term pregnancy helped drive down the risk by 15 per cent, and that the reduced risk was seen in up to as many as eight additional pregnancies.
No benefit was found for women who had twin or multiple births.
Endometrial cancer, which begins in the uterus, is the fifth most commonly diagnosed cancer for Australian women. According to Cancer Australia, more than 3,100 women were diagnosed last year.
It mostly affects women aged over 55 and symptoms include abnormal vaginal bleeding - including after menopause or bleeding between periods, pelvic pain and/or painful sex, however some women experience no symptoms.
Rates of endometrial cancer have been increasing over the past two decades and the researchers said it was vital to get a better understanding of any drivers.
More than 30 international studies were examined for the research, involving date from 16,986 women with endometrial cancer and 39,538 women who have never had the cancer.
Lead author of the study, Associate Professor Susan Jordan (now at the University of Queensland) said the study challenged the belief that it was the hormone levels in the third trimester which provided a protective effect against the cancers.
"Our analysis in this large group of women shows that while a full-term pregnancy is associated with the greatest reduction in risk for endometrial cancer, even pregnancies that end in the first or second trimester appear to provide women with some protection," Associate Professor Jordan said.
"This suggests that very high progesterone levels in the last trimester of pregnancy is not the sole explanation for the protective effect of pregnancy. If women who experience miscarriage have a seven to nine per cent reduced risk of endometrial cancer then early pregnancy factors may also be playing a protective role against this disease."
Having boys was also found to lower the risk, with mums who had sons determined to be at a reduced risk than women who had only daughters.
More research to identify other factors involved in the protective effect was needed, researchers said.
"This raises the need for more research to identify other factors that underlie this protective effect."