The contraceptive pill is arguably one of the biggest advancements in women's health of the 20th century. A recent study suggests that the birth control pill not only gave women the opportunity to use their reproductive rights, but has also prevented hundreds of thousands of cases of womb cancer.

According to new research, published in the journal The Lancet Oncology, the pill reduces the incidence of womb cancer, which is also known as uterine or endometrial cancer. This form of cancer affects the female reproductive system and usually occurs after menopause. Womb cancer often begins in the inner lining of the womb, which is called the endometrium. The most common symptom is abnormal vaginal bleeding.

To investigate the link between the pill and the subsequent risk of womb cancer, researchers analyzed data from 36 studies that included 27,276 women with womb cancer and 115,743 women without it. Researchers note in the study that this is almost all available data on the effect of the pill on endometrial cancer.

They found that the pill had prevented an estimated 400,000 womb cancer cases in the past 50 years, including 200,000 in the last decade. The pill reduced the risk of womb cancer by around a quarter for every five years on the pill and those who took birth control pills for 10 years also cut the risk of ovarian cancer in half.

Researchers are unsure of the exact mechanisms of how protection against womb cancer could be conferred, but speculate it plays a role in reducing exposure to unopposed oestrogen during the follicular phase of the menstrual cycle, which could prevent rapid oestrogen-induced cell growth.

“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,” co-author Valerie Beral, from the University of Oxford, said in a statement to the press.

The American Cancer Society estimates there are about 54,870 new cases of womb cancer and around 10,170 deaths in the U.S. each year. According to Cancer Research UK, womb cancer is the fourth most common cancer in Europe for females, and the tenth most common cancer overall.

Researchers note that although estrogen levels in birth control pills have decreased, with pills in the 1960s containing more than double the level of estrogen than pills in the 1980s, the level of protection stayed the same even in lower doses.

Different doses and versions of the pill still come with some risks, such as blood clots, strokes and heart attacks, researchers warn. Beral told The Guardian that even though the evidence for the pill’s protection falls short of it being prescribed to prevent cancer, studies like this are important as “women need to be more aware of the unintended benefits and the risks of oral contraceptives, so that they can make informed decisions.”