Pill and pregnancy have biggest effects on ovarian cancer risk
Overall, women who took the Pill for any length of time had an approximately 15 per cent lower risk of ovarian cancer than those who never took it at all - and the risk reduced further in women taking it for longer.
The study, published in the British Journal of Cancer, looked at how various reproductive factors affected the risk of ovarian cancer.
Taking the Pill for more than 10 years had the biggest impact on reducing the risk of the disease, followed by getting pregnant and having more than one child.
Among women who used the pill for a year or less, the risk of developing ovarian cancer was around 28 per 100,000 per year. For women who took the pill for at least 10 years, this risk fell by about half to around 15 per 100,000 per year.
Women who had ever been through a full-term pregnancy had a 29 per cent lower ovarian cancer risk compared with women who had never been pregnant.
Among women who had never been pregnant, the risk of developing ovarian cancer was 34 per 100,000 per year. For women who had at least one full-term pregnancy, this risk fell to around 24 per 100,000 per year.
And the bigger the family, the greater the benefit among women who have children, each additional child lowered the risk of ovarian cancer by a further eight per cent.
The research is part of ongoing work by the European Prospective Investigation of Cancer (EPIC) a Cancer Research UK co-funded study and one of the largest studies into the links between diet, lifestyle and cancer.
Naomi Allen, a Cancer Research UK epidemiologist, based at the University of Oxford who works on the EPIC study, said: Ovarian cancer is difficult to detect and so prevention is key to saving women suffering from this disease. These results are important because most women dont know that taking the Pill or getting pregnant can help reduce their risk of ovarian cancer later on in life.
Research has not yet established how factors like the Pill and pregnancy reduce ovarian cancer risk. One theory is that taking the Pill or getting pregnant changes the level of hormones that can affect a womans risk of the disease.
While the Pill has a protective effect against ovarian cancer, women taking the Pill have an increased risk of breast cancer while using it, but which disappears after use has stopped.
Ovarian cancer is the fifth most common cancer in women in the UK with over 6,500 cases diagnosed each year.
The link between ovarian cancer and pregnancy and using oral contraceptives is already well established - this study adds weight to the evidence.
Dr Richard Edmondson, a Cancer Research UK women's cancer expert based at the University of Newcastle, said: "Women may be reassured to know that the oral contraceptive is not only an effective contraceptive but can have the added benefit of reducing their risk of ovarian cancer.
This is however balanced against a slightly increased risk of developing breast cancer. To put this in context, it is estimated that if 100,000 women use the pill for ten years or more there will be 50 more breast cancers than would have otherwise occurred, but 12 fewer ovarian cancers.
This may be particularly important for women with an increased risk of ovarian cancer in their family.
Sara Hiom, director of health information at Cancer Research UK, said: These days it is not uncommon for women to have fewer children or none at all. Women tend to be unaware that these reproductive factors have a protective effect on their risk of ovarian cancer.
Nobody can expect women to start living like their Victorian counterparts to reduce their risk of the disease. But there are other things that can be done to lower the risk of ovarian cancer like stopping smoking and maintaining a healthy weight.
As with most cancers, the risk of developing ovarian cancer increases with age - most cases are in women who are past their menopause. Inherited faulty genes can also play a significant role, and women who think they may have a family history should discuss this with their doctor.
Treatment for ovarian cancer is better if the disease is caught as early as possible. So all women should be aware of the signs of ovarian cancer like pain in the lower tummy, bloating, increased tummy size, difficulty eating or feeling full. If these symptoms are new and happen on most days then its worth getting checked out by your doctor without delay.