A woman's chance of developing different types of ovarian cancer could depend on whether she is sterilised or has kids, a new study suggests.
Experts already know that women with children have a lower risk of the disease, and that having more children cuts the risk significantly, PA reports.
But now researchers have been able to work out the risks of different types of ovarian cancer according to whether a woman has children.
They also found that women who had their fallopian tubes cut during sterilisation have a lower risk of cancer - possibly because some ovarian cancers start in the fallopian tubes.
Around 7,100 women are diagnosed with ovarian cancer in the UK each year. It is the fifth most common cancer in women, after breast, lung, bowel and womb cancer.
The four most common types of ovarian cancer are serous, mucinous, endometrioid and clear cell tumours.
In the new study, experts found that women with one child had around a 20% reduction in the risk of developing any type of ovarian cancer compared to women without children.
They also had around a 40% reduced risk of endometrioid and clear cell tumours.
Each extra child then led to an estimated 8% reduction in the overall risk of ovarian cancer.
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Women who were sterilised also had a 20% lower overall risk of ovarian cancer compared with those who had not undergone surgery.
The risk was about 20% lower for high-grade serous tumours - the most common type of ovarian cancer - and about 50% lower for endometrioid and clear cell tumours.
The study, presented at the National Cancer Research Institute (NCRI) cancer conference in Liverpool, involved data from more than 8,000 women with ovarian cancer.
Cancer Research UK-funded scientist, Dr Kezia Gaitskell, from the University of Oxford, said: "In the last few years, our understanding of ovarian cancer has been revolutionised by research showing that many cases may not in fact come from the ovaries.
"For example, many high-grade serous tumours - the most common type - seem to start in the fallopian tubes, while some endometrioid and clear cell tumours may develop from endometriosis.
"We think that the significant reduction in risk among women with one child compared to women without children is likely to be related to infertility, as there are some conditions - such as endometriosis - that may make it harder for a woman to become pregnant, and which may also increase her risk of these specific types of ovarian cancer.
"For the reduced risk seen among women with tubal ligation (sterilisation) - it could be that tubal ligation acts as a barrier to help prevent the abnormal cells that might cause these tumours passing through the fallopian tubes to the ovaries.
"Our results are really interesting, because they show that the associations with known risk factors for ovarian cancer, such as childbirth and fertility, vary between the different tumour types."
Annwen Jones, chief executive of Target Ovarian Cancer, said: "This research provides a further insight into the different types of the disease, as well as where the disease begins.
"It's important that this research paves the way for us to understand the mechanisms behind these findings, so that we can some way develop a way to lower risk of ovarian cancer for all women. This research could also help us to develop new, more effective, treatments for ovarian cancer."