Women given a 12-month supply of oral contraceptives are less likely to have an unintended pregnancy than if they need to get refills every three months, US research suggests.

The authors used a computer model to look at the impact of reducing the number of times a woman would have to refill her contraceptive prescriptions on the risk of unplanned pregnancies and cost, using data from more than 24,000 women who had served in the military.

The data showed that 43% of the women who received their oral contraceptives in the stipulated three month increments experienced at least one gap of at least seven days between refills over the course of a year, the authors wrote in JAMA Internal Medicine.

Annually, 149 unintended pregnancies were expected per 1000 women with the 12-month option.