Abortion rates dropped steeply over the course of 25 years in countries like the U.S., new research shows, though the same trend did not occur in still-developing nations.

A study published Wednesday in the journal The Lancet measured abortion incidence between 1990 and 2014, finding that the number of abortions per 1,000 women of childbearing age in developed countries dropped from 46 to 27 over the time frame, while in developing countries it dropped from 39 to just 37.

The annual number of abortions worldwide over the period rose by almost 6 million to 56.3 milllion – an increase researchers attributed to growth of the global population. The overall abortion rate for 2010-2014 was 35 per 1,000 women – down from 40 per 1,000 women two and a half decades ago.

Researchers also found that during the last five years evaluated, unmarried women were less likely to have an abortion than married women. Roughly 73 percent of abortions were obtained by married women.

The World Health Organization and the Guttmatcher Institute – a U.S. research and policy organization that supports abortion rights – partnered on the study.

In a statement, the study's lead author, Gilda Sedgh of the Guttmacher Institute, said the trends show that women in regions that saw a decline in abortion rates have greater access to contraception.

"Our findings indicate that in many developing regions, women lack the contraceptive services they need and are unable to prevent pregnancies they do not want to have," Sedgh said.

When countries were grouped according to their abortion laws, researchers found no significant difference in recent abortion rates between countries where the procedure was legal and where it was restricted.

Where abortion was illegal or permitted only to save a woman's life, the rate was 37 per 1,000. Where it was available on request, the abortion rate was 34 per 1,000.

