LIVERPOOL, ENGLAND—The more children a woman has or whether she has undergone tubal ligation lowers her risk of different types of ovarian cancer to different levels, according to new research presented at the 2015 National Cancer Research Institute (NCRI) Cancer Conference.

Data was collected from more than 8,000 women with ovarian cancer as part of the UK Million Women Study. Researchers then examined the risk of the 4 most common types of ovarian cancer—serous, mucinous, endometrioid, and clear cell tumors—in women with different childbearing patterns.

For ovarian cancer overall, women with one child had an approximately 20% reduction in risk compared with women with no children, and an approximately 40% reduction of endometrioid and clear cell tumors. Each additional birth then offered an estimated 8% reduction in the overall risk of ovarian cancer.

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Researchers also compared risk between those women who underwent tubal ligation. Women who underwent the procedure had a 20% lower overall risk for ovarian cancer. The risk was approximately 20% lower for high-grade serous tumors, the most common type of ovarian cancer, and approximately halved for endometrioid and clear cell tumors.

“In the last few years, our understanding of ovarian cancer has been revolutionized by research showing that many cases may not in fact come from the ovaries,” said lead researcher and pathologist Kezia Gaitskell, MD, of the Cancer Epidemiology Unit, University of Oxford. “For example, many high-grade serous tumors, the most common type, seem to start in the fallopian tubes, while some endometrioid and clear cell tumors 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.

Gaitskell stated that the reduced risk for women after tubal ligation may occur because the procedure creates a barrier that prevents abnormal cells that might cause ovarian tumors from passing through the fallopian tubes to the ovaries.

Professor Charlie Swanton, MD, PhD, Chair of the 2015 NCRI Cancer Conference, said, “We’ve known for some time that the number of children a woman has, and her use of contraception, can influence her risk of ovarian cancer, so this research provides important further detail about different types of the disease.

“Ovarian cancer, like many other cancers, is not one disease, but different diseases that are grouped together because of where they start. 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.”