The American Cancer Society estimates that over 61,000 new cases of endometrial cancer will be diagnosed in the U.S. in 2017, with nearly 11,000 of those being fatal. Also known as womb or uterine cancer, the disease mainly affects post-menopausal women.

That's the bad news. The good news is that, in addition to preventing unwanted births, use of oral contraceptives has also proven pretty effective at preventing womb cancer. Researchers from the Collaborative Group on Epidemiological Studies on Endometrial Cancer estimate that in the past 50 years about 400,000 cases of endometrial cancer have been prevented by oral contraceptive use in high-income countries, including about 200,000 in the last decade.

"The strong protective effect of oral contraceptives against endometrial cancer – which persists for decades after stopping the pill – means that women who use it when they are in their 20s or even younger continue to benefit into their 50s and older, when cancer becomes more common," explained study author Professor Valerie Beral, from Oxford University.

She added "Previous research has shown that the pill also protects against ovarian cancer. People used to worry that the pill might cause cancer, but in the long term the pill reduces the risk of getting cancer."

The scientists examined data from over 22,000 women with endometrial cancer in 26 countries. The research showed how every 5 years of oral contraceptive use reduces the risk of womb cancer by about a quarter. In high-income countries, 10 years of oral contraceptive use reduces the risk of developing endometrial cancer before age 75 from 2.3 to 1.3 cases per 100 users.

Back in the 1960s, estrogen doses in oral contraceptives were appreciably higher than they are today – more than double the dose from the 1980s. Yet the reduction in endometrial cancer risk was at least as great for women who used the pill during the 1980s as for those who used it in earlier decades. This suggests that the amount of hormones in the lower-dose pills is still sufficient to reduce the incidence of endometrial cancer, according to the authors.

The proportional risk reduction did not vary substantially by women's reproductive history, amount of body fat, alcohol use, tobacco use, or ethnicity.

The results of the study were published in Lancet Oncology.