It is already known that having children can reduce a woman's risk of developing ovarian cancer. However a new study has found that the number of children reduces the risk of different types of the disease to different levels.

UK researchers studied health data on over 8,000 women with ovarian cancer. They looked specifically at the risk of the four most common types of the disease - serous, mucinous, endometrioid and clear cell tumours.

They found that women with one child had a 20% reduced risk of developing ovarian cancer overall compared to women with no children. However when it came to endometrioid and clear cell tumours, the risk was 40% lower among women with one child compared to those without children.

Each additional birth appeared to offer around an 8% reduction in the overall risk of ovarian cancer.

"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," commented lead researcher, Dr Kezia Gaitskell, of the University of Oxford.

Meanwhile, the researchers also found that women who had their fallopian tubes cut or clipped had a 20% reduced risk of developing ovarian cancer overall.

Known as tubal litigation or sterilisation, this is a surgical procedure that results in permanent contraception.

The study showed that the risk of high-grade serous tumours, which is the most common type of ovarian cancer, was around 20% lower among those who had been sterilised, while their risk of endometrioid and clear cell tumours was around 50% lower.

"For the reduced risk seen among women with tubal ligation, it could be that this 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," Dr Gaitskell said.

Details of these findings were presented at the 2015 National Cancer Research Institute (UK) Cancer Conference in Liverpool.