What we're talking about is abortion -- a subject that is often politicized and stigmatized but, experts say, cannot be ignored when discussing women's health.
Abortion rates in the developed world have fallen to a historic low, the researchers say, but that's not the whole picture.
Some "300,000 women die during pregnancy and childbirth each year," and "as many as 1 in 7 maternal deaths is caused by unsafe abortion," said Dr. Richard Horton, editor in chief of The Lancet and former co-chairman of a U.N. expert review group on women and children's health.
To truly care about maternal health, he says, means caring about safe abortions. And he and others hope the report serves as "a wake-up call" to prioritize access to both contraception and safe abortion services.
At a glance
The researchers looked at total numbers for each year between 1990 and 1994 and between 2010 and 2014. The numbers were averaged for those five-year periods to help draw comparisons. Among their findings:
56.3 million: Number of abortions worldwide on average each year in 2010-14, an increase of 5.9 million from 1990-94. This increase, scientists say, is due to population growth.
35: The average number of abortions annually and globally per 1,000 women age 15-44 in 2010-14, a decrease from 40 per 1,000 in 1990-94.
46 to 27: The decline in the abortion rate per 1,000 women in the developed world over this 25-year period, which the researchers said is a historic low.
39 to 37: The decline in the abortion rate per 1,000 women in the developing world. The researchers say this slight decline is statistically insignificant.
88%: The percentage of abortions that occur in the developing world.
1 in 4: Pregnancies that ended in abortion around the world in 2010-14.
Married vs. unmarried
Married women, classified for the study's purposes as those married to or cohabiting with a man, had 73% of all abortions during 2010-14.
This difference between married and unmarried rates of abortion was most significant in Europe, where 38 per 1,000 married women had an abortion in a year and 16 per 1,000 unmarried women did.
The region where the most married women had abortions was Latin America, where 49 out of 1,000 women had one annually.
Unmarried women represent 35% of all women of reproductive age, the study says, and an estimated 15 million unmarried women had abortions each year between 2010 and 2014.
Regions with the biggest changes
The biggest decline in abortion rates over the past 25 years was in Eastern Europe, where access to contraception was limited under the Soviet regime. Annual rates of abortions there dropped from 88 to 42 per 1,000 women from 1990-94 to 2010-14. In more stark terms, going back to 1990-94, 54% of pregnancies in Eastern Europe ended in abortion. Twenty-five years later, that percentage dropped to 38% -- which still ranks among the highest globally.
The only region to have a higher rate of abortions was the Caribbean, where 39% of pregnancies ended in abortion.
That Caribbean figure was shaped in large part by Cuba, which, like Eastern Europe, had limited access to contraception during the Soviet era, explained Gilda Sedgh, a principal research scientist at the Guttmacher Institute and one of the authors of the study.
"Many had to rely on abortion to avoid unplanned births," Sedgh said. "While both regions have made great strides in women's access to and use of contraception, they are still not on par with the rest of the developed world in this respect."
In Europe, just one region saw an increase in abortion rates, and that was Western Europe. This can be attributed to the increased size of the foreign-born population, which may not yet know where or how to best access reproductive health and contraceptive services, Sedgh said.
In Latin America, where abortions are highly restricted, if not completely illegal, there was the greatest increase in the percentage of pregnancies ending in abortion. While 23% of pregnancies ended in abortion each year in 1990-94, that figure increased to 32% over 25 years
How restrictions can backfire
Restricting or legally banning access to abortions does not prevent them from happening. In Latin America, the report showed, an average of 6.5 million abortions occurred each year between 2010 and 2014.
Furthermore, the very places with restrictive abortion laws also have higher levels of unmet contraceptive needs. And for women in developing countries, where there's an increased desire for smaller families, this lack of family planning services means more unwanted pregnancies that will end in abortion, said Sedgh.
When abortions are illegal, women who want them often turn to clandestine measures and often unsafe abortions, creating new problems.
Studies to measure the proportion of abortions deemed unsafe are under development, but nearly $300 million "is spent each year on treating complications from unsafe abortions," said co-author Dr. Bela Ganatra of the World Health Organization. "Investing in modern contraceptive methods would be far less costly to women and society than having unwanted pregnancies and unsafe abortions."
In the developing world, an estimated 6.9 million women had to be treated for complications related to unsafe abortions in 2012, the report showed. And with the stigma attached to abortion, it's believed as many as 40% of women who needed care did not receive it.
Abortion itself, when performed up to standards, is not inherently unsafe, Sedgh said.
Less than 0.05% of abortions in the developed world lead to any complications, she said. And that strongly juxtaposes with the reality in the developing world, where 20% of abortions result in complications
"No woman wants an unwanted pregnancy," Horton, of The Lancet, added. "But when that event happens, it's absolutely essential that health services respond with compassion, understanding and practical help."