New research further correlates pregnancy and fallopian tube ligation surgery to a decreased risk of developing ovarian cancer, and links differing levels of that risk to different types of ovarian cancers, including those that may develop from endometriosis.  The research study, titled “Childbearing patterns and ovarian cancer in the Million Women Study: variation by histological subtype” and part of the UK Million Women Study carried out by Cancer Research UK, was presented at the 2015 National Cancer Research Institute (NCRI) Cancer Conference.

Reduced risk for ovarian cancer has been reported for women that have given birth. Ovarian cancer is not a homogeneous disease, as different types of tumors with distinct histological and molecular features suggest different origins. Despite the evidence of different cancer subtypes, few epidemiology studies have been made to determine risk variation.

The Million Women Study included data collected from more than 8,000 women with ovarian cancer to evaluate the risk of developing the most common types of this malignancy, such as serous, mucinous, endometrioid and clear cell tumors (subtypes of epithelial ovarian cancer). Women with one child had an overall 20% risk reduction of developing ovarian cancer and a 40% risk reduction of developing endometrioid and clear cell tumors when compared to women without children. The risk of developing ovarian cancer dropped an estimated 8% for each additional birth.

Researchers also looked at the influence of tubal ligation, a surgical procedure where the fallopian tubes are clamped, blocked or sealed, with the risk of developing cancer. Women who had undergone the procedure had an overall 20% lower risk of developing ovarian cancer. More specifically, they a 20% lower risk of developing high-grade serous tumors (most common type of ovarian cancer) and a 50% decreased risk of endometrioid and clear cell tumors compared to women who did not have the procedure.

“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,” Dr. Kezia Gaitskell, Cancer Research UK funded lead researcher and pathologist based at the Cancer Epidemiology Unit, University of Oxford, said in a press release. “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 — 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,” Dr. Gaitskell added.

Professor Charles Swanton, Chair of the 2015 NCRI Cancer Conference, said, “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.”