Contraception saves more than a quarter of million lives each year, show results.
In 2008, 355,000 women died while giving birth or from illegal or dangerous abortions, a study published by The Lancet said.
But more than 250,000 deaths were averted that year because contraception reduced unwanted pregnancies, it said.
"If all women in developing countries who want to avoid pregnancy use an effective contraceptive method, the number of maternal deaths would fall by a further 30 percent," according to the research.
The paper, led by John Cleland, a professor at the London School of Hygiene and Tropical Medicine, appears in The Lancet on the eve of a "London Summit on Family Planning," promoted by the Bill and Melinda Gates Foundation.
It is campaigning for the rights of 120 million women and girls to have access to family planning.
"Increasing contraceptive use in developing countries has cut the number of maternal deaths by 40 percent over the past 20 years," said the paper.
It also pointed to the benefits for child health and pregnancies that are planned and spaced out.
"In developing countries, the risk of prematurity and low birthweight doubles when conception occurs within six months of a previous birth, and children born within two years of an elder sibling are 60-percent more likely to die in infancy than are those born more than two years after their sibling."
The world's population reached seven billion last year and is likely to climb to around 9.3 billion by 2050, and more than 10 billion by 2100, according to UN estimates.
Demographic growth will be overwhelmingly concentrated in the poorest countries, especially Africa.