pregnancy and endometrial cancer

The specifics of a long-known protective association between pregnancy and endometrial cancer were studied in a recent Danish investigation.

Endometrial cancer is the most common gynecological cancer in women, and as a result, much research has been put into understanding the risk factors. A strong association has previously been found between pregnancy and endometrial cancer – with pregnancy decreasing the risk of developing endometrial cancer.

Pregnancy reduces the total number of menstrual cycles a woman will have in a lifetime, and this is an important factor in the protective effect of pregnancy. This idea is backed up by the observation that women on the contraceptive pill, and therefore having fewer menstrual cycles also seem to be protected against endometrial cancer.

A group of researchers in Denmark aimed to add more specific details to these observations and investigated whether the number of months spent pregnant matters. For example, would a woman with four pregnancies have a greater reduction in the risk of endometrial cancer compared to a woman with one pregnancy? Their findings were published in the British Medical Journal.

What they found turns the assertion of the number of menstrual cycles being the factor at play, on its head. The researchers found that, in fact, the number of months of being pregnant made no difference. After studying data from over two million women, with a total of almost four million pregnancies, over a period of 67 years, the researchers found that even women who had abortions as early as eight weeks into pregnancy still seemed to have the protective effect against endometrial cancer.

The researchers rethink their ideas on the association between endometrial cancer and pregnancy and suggest, that actually, the protective effect may be from a biological process occurring in the first few weeks of pregnancy. As an observational study, the researchers cannot use the data to prove this theory. Indeed, there may be a confusing factor in the data, in that, women who had abortions may have also been on the contraceptive pill and obtained a protective effect from that; this information was not included as part of the study. However, the researchers conclude that their results strongly suggest that factors early in pregnancy are responsible for a reduction in the risk of endometrial cancer.

Written by Nicola Cribb MA VetMB DVSc Diplomate ACVS

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Reference: Husby A, Wohlfarht J, Melbye M. Pregnancy duration and cnacner risk: nationwide cohort study. BMJ 2019;366:14693

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