Abortion rates in developed countries have been falling steadily since 1990, but rates in developing countries have stayed roughly the same, a new study said.

The study, published Wednesday in The Lancet, found that the worldwide abortion rate dropped slightly from 1990 to 2014, to 35 from 40 abortions per 1,000 women of childbearing age. The decline is largely due to developed countries, where abortion rates dropped from 46 to 27 per 1,000. The United States has among the lowest rate, about 17 per 1,000.

In developing countries, the rate has changed little, to 37 per 1,000, from 39. The difference between developed and developing countries is directly correlated with contraception use, said the lead author, Gilda Sedgh, the principal research scientist at the Guttmacher Institute, a reproductive health organization that supports abortion rights. It conducted the study with the World Health Organization.

Dr. Sedgh said the gap in contraception use was much less about access than it used to be. “We have made contraception available to a lot more women,” she said. “In the 1980s, when women who wanted to avoid getting pregnant weren’t using a method, their most common reason was lack of access. Now that’s their least common reason, less than 5 percent.”