07/09/2016 2:29 PM AEST | Updated 11/09/2016 1:50 PM AEST

The Contraceptive Pill Has Lowered Ovarian Cancer Deaths Globally

We're just not entirely sure how it works.

Nikita Sobolkov
The pill lowers the risk of ovarian cancer, but it's not perfect.

Worldwide, the rate of ovarian cancer deaths is declining as more women take the contraceptive pill, but it's more complicated than suggesting one eliminates the other.

A global study, published in cancer journal the Annals of Oncology, found Australia's ovarian cancer death rate declined by 12 percent in a decade, while the decline was 16 percent in the U.S. and 8 percent in Canada.

Researchers noted the declines matched the uptake of the contraceptive pill decades ago and in countries such as Spain, Italy and Greece, where oral contraceptive use started much later and was less widespread, the decline was not so pronounced.

Science is yet to explain the mechanism of how the pill protects ovaries against cancer. QIMR Berghofer Medical Research Institute head of gynecological cancers Penny Webb explains what we know about ovarian cancer.

What is ovarian cancer?

"That sounds like an easy question but actually it's not," Webb told HuffPost Australia.

"Ovarian cancer is so called because it's always been assumed it started in the ovaries but we now think quite a lot of what we call ovarian cancer might start in the fallopian tubes.

For all intents and purposes, it's a group of cancers that generally affect the ovaries."

Stringer Germany / Reuters
Actress Angelina Jolie had her ovaries and fallopian tubes removed to avoid the risk of ovarian cancer.

The Cancer Council reports 1400 Australian women are diagnosed with ovarian cancer each year, making it the ninth most common cancer in women.

This research suggests the contraceptive pill is partly to thank for the dropping death rate, do you agree?

"It's a very likely explanation," Webb said.

"We know for sure that women who have used the pill have a much lower risk than women who haven't.

"The same is true for women who have had more children. Both the pill and having children stop women ovulating. There are suggestions it may have something to do with the physical process of ovulation, but then there are also the hormonal changes.

"If you're taking the pill, the body's hormone environment is obviously very different than if you're ovulating and the same is true for pregnancy."

University of Sydney Professor Deborah Marsh told HuffPost Australia it might be to do with inflammation.

"There's certainly some theories, like the incessant ovulation hypothesis that suggests there's a lot of inflammation associated with ovulation," Marsh said.

"Inflammation is related to a higher risk of cancer and the pill gives the body a break from all that on/off process of ovulation.

Science hasn't exactly nutted out how these protective mechanisms work but it seems to have something to do with inflammation."

How long do you need to take the pill for to get the protective benefits?

"The interesting thing is that women usually take the pill in their 20s, 30s and 40s, but most ovarian cancers occur in women in their 50s, 60s and 70s," Webb said.

Justin Case
Women taking the pill in their 20s saw an effect in their 60s.

"Whatever the pill is doing to young women is working when they're older.

"The longer you take the pill, it seems the lower your risk but I don't know whether there's a point whereby going beyond it wouldn't make any difference."

What about the negative effects of the pill?

"Nothing's perfect I guess," Webb said.

"The pill certainly seems to greatly decrease a woman's risk of ovarian and endometrial cancers in the uterus but there are also adverse effects. It also seems that while women are taking the pill, it slightly increases the risk of breast cancer, but that seems to drop off pretty quickly once they stop.

"I personally don't think we should go out and tell women to take the pill to avoid ovarian cancer. Having said that, the pill is an excellent contraceptive and women can make their own choice of what works for them."