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Researchers have developed a once-a-month oral contraceptive pill that they say would offer women more choice and control over their fertility.

Once swallowed, the pill remains in the stomach for weeks, slowly releasing hormones to prevent a pregnancy.

It is specially designed to resist immediate attack by stomach acid.

A US team, funded by The Bill and Melinda Gates Foundation, has tested it in pigs and hopes human trials can start within a few years.

The researchers say it would be a good option for women who want to take a tablet for birth control, but are worried about remembering to take a daily dose.

How effective is the Pill?

The conventional Pill or combined oral contraceptive is used by millions of women around the world.

It should be 99% effective, meaning fewer than one in every hundred women using it will get pregnant.

But studies suggest nearly half of users will miss the odd dose or sometimes take their pill at the wrong time.

That makes the pill closer to 91% effective, meaning about nine in every hundred women using it will get pregnant in any given year.

There are other longer-lasting contraceptive choices already available - including bi-monthly injections, or patches that need changing weekly - but no once-a-month oral pill.

How does the once-a-month pill work?

The prototype is a star-shaped drug delivery system packaged into an easy-to-swallow dissolvable capsule no bigger than a regular fish oil tablet.

Once it reaches the stomach, the star unfolds like a flower and starts doing its work of steadily releasing contraceptive hormones housed on its six arms.

The star is too big to immediately exit the stomach and will remain there for weeks until it has finished its job and can be broken down and excreted from the body.

Dr Giovanni Traverso, from Harvard Medical School, who developed the prototype with colleagues at Massachusetts Institute of Technology, said: "Our studies suggest there shouldn't be any problem with blockages or digesting and passing food. We take safety very seriously."

The trial findings are published in the journal Science of Translational Medicine.

A biotech company called Lyndra is now developing the star contraceptive pill with more funds from The Gates Foundation.

It has already started testing the same star-shaped design in patients, and wants to see if it can safely and reliably deliver other drugs, such as malaria therapies, too.

What do experts think?

Dr Diana Mansour, from the Faculty of Sexual and Reproductive Healthcare, said: "The concept of a monthly oral contraceptive pill is attractive and has the potential to broaden contraceptive choice.

"In theory, a monthly pill may be more effective than current oral contraceptives, which women are required to take every day.

"However, the development of such a novel contraceptive is still in its early stages. We look forward to further research in this area."

Anatole Menon-Johansson, from the charity Brook, said: "This is an exciting breakthrough for contraceptive options, and expanding the range of methods available is certainly a positive.

"However, additional work is required to ensure that the dose can be delivered at a sufficient level to suppress ovulation, and then this approach will need to be tested in humans."