Women's risk of developing ovarian cancer is significantly reduced by having a baby, and the risk drops further still for each subsequent child a mother gives birth to, a new study has found.

Data collected from 8,000 women with ovarian cancer as part of the UK Million Women Study has revealed that women who give birth to one child have about a 20 percent reduction in risk compared to women who do not have children, and their risk of developing endometrioid and clear cell tumours is 40 percent lower.

Women who have more than one child are even less likely to develop ovarian cancer, with each subsequent baby delivering an estimated eight per cent reduction in overall risk.

But while the findings at first glance seem to suggest that having bubs is an actionable cancer deterrent, it's not that simple. The researchers also looked at women who had had their fallopian tubes cut or clipped surgically (called tubal ligation or sterilisation), which provides a permanent means of contraception.

Women who underwent this procedure also had a 20 per cent lower overall risk of ovarian cancer, providing further evidence that the causes of ovarian cancer are more complex than was once believed.

"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," said Kezia Gaitskell from the Cancer Epidemiology Unit at the University of Oxford and lead researcher on the study. "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."

The findings, which were presented at the 2015 National Cancer Research Institute conference in the UK this week, don't necessarily mean that choosing to have more children will protect you from cancer, but statistically speaking, women who have more children are less at risk.

"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," said Gaitskell.

The researchers also theorise that tubal ligation may lower cancer risk as it serves as a barrier that effectively stops abnormal cancer-causing cells from passing through the fallopian tubes to the ovaries.

"Ovarian cancer – like many other cancers – is not one disease, but different diseases that are grouped together because of where they start," said Charles Swanton, chair of the conference, who was not involved with the research.

"It's important to know what affects the risk of different types of ovarian cancer and what factors impact this. We now need to understand the mechanisms behind these findings to develop some way to extend this lower risk to all women, regardless of how many children they have."