Out-of-pocket spending on most major birth control methods fell sharply in the months after the Affordable Care Act began requiring insurance plans to cover contraception at no cost to women, a new study has found. Spending on the pill, the most popular form of prescription birth control, dropped by about half in the first six months of 2013, compared with the same period in 2012, before the mandate took effect.

The study, by health economists from the University of Pennsylvania, analyzed health insurance claims from a large private insurer with business in all 50 states and the District of Columbia. It evaluated the effect of the Affordable Care Act, the biggest piece of social legislation in decades, on women’s pocketbooks. It estimated that savings from the pill alone were about $1.4 billion in 2013.

Cost has long been a major obstacle to women getting birth control, and declines in what they pay for contraceptives have the potential to increase access and reduce unplanned pregnancies. About half of the 6.6 million pregnancies a year in the United States are unintended, far higher than in most developed countries.

The study, published online in Health Affairs on Tuesday, was not able to definitively establish whether the law drove women’s falling expenditures on birth control, but experts said the magnitude and timing of the decline suggested that it was.