In 2003, the latest year for which full figures are available, 42 million abortions were carried out around the world, compared with 46 million in 1995, according to the paper published by The Lancet next Saturday.
For every 1,000 women aged between 15 and 44 in 2003, 29 had an abortion, down from 35 in 1997, it said.
"Overall abortion rates are similar in the developing and developed world, but unsafe abortion is concentrated in developing countries," it notes.
"Ensuring the need for contraception is met and that all abortions are safe will reduce maternal mortality substantially and protect maternal health."
Abortion rates were lowest in Western Europe (12 pregnancy terminations per 1,000 women) but highest in Eastern Europe, where the rate was 44 abortions per 1,000 women. In the United States and Europe, it was 21 per 1,000, while in Asia and Africa, the rate was 29 per 1,000.
Forty-eight percent of all abortions worldwide were unsafe, and more than 97 percent of unsafe abortions took place in developing countries.
On the basis of the 2003 data, on average 90 percent of women worldwide will have had an abortion before the age of 45, the study calculates.
This varies, though, between many women who will have had multiple terminations and many who will have had none at all, note the authors, led by Gilda Sedgh of the Guttmacher Institute in New York and Iqbal Shah of the World Health Organisation (WHO).