Uterine cancer is common. Among women in the U.S., it’s the fourth most common type of cancer —about one in 35 women will be diagnosed with it at some point in their lives. (BTW, uterine cancer is also sometimes called endometrial cancer since it often starts in the lining of the uterus, the endometrium.)

But what does uterine cancer have to do with birth control?

The pill protects against uterine cancer

Actually, we’ve known for a long time that the pill protects women from this type of cancer . But if you take the pill in your 20s and this kind of cancer comes along 40 years later, it’s complicated to measure just how much protection the pill offers. Now an international group of researchers have tackled this complicated question .

They found that:

• Overall, women who used the pill had a 30% reduction in cancer of the uterus.

• The longer women used the pill, the less likely they were to develop uterine cancer. Women who used the pill for less than 5 years had a 20% reduction, while women who used it for 10 to 15 years had a 50% reduction.

• The protective effect of the pill lasted for decades. Even when 30 years had passed since the last time women used the pill, they still had substantial decreases in cancer of the uterus.

Equal opportunity protection

There are some other factors linked to increased risk for uterine cancer. But this study showed that the pill trumped them all. Women’s smoking or drinking habits, how many children they had, their race/ethnicity, their weight, and when their periods started didn’t change the overall protective effect of the pill.

Since so many women have used the pill since its creation in the 1960s, the researchers estimate that over 400,000 cases of uterine cancer have been prevented around the world. How’s that for side effects?