Researchers tracked more than 9,000 poor or uninsured women. When price wasn't an issue, more women used the most effective contraceptive methods, like an IUD implant, a goof-proof method that typically costs hundreds of dollars upfront to insert. Dr. Jeffrey Peipert of Washington University in St. Louis said these women experienced far fewer unintended pregnancies as a result.
For teen pregnancy, there were 6.3 births per 1,000 teenagers in the study. This is a striking difference to the national rate of 34 births per 1,000 teens in 2010.
Women in the study showed substantially lower rates of abortion than women in the metro area and nationally: 4.4 to 7.5 abortions per 1,000 women in the study, compared with 13.4 to 17 abortions per 1,000 women overall in the St. Louis region. That's lower than the national rate, too, which is almost 20 abortions per 1,000 women.
Peipert's team said if the study was expanded, one abortion could be prevented for every 79 to 137 women given a free contraceptive choice.
The findings of the study, which was published in the journal Obstetrics and Gynecology, ran from 2008 to 2010. The results come as millions of U.S. women are beginning to get access to contraception without co-pays under President Barack Obama's health care law.
Experts say the results of this study foreshadow that policy's potential impact.
"As a society, we want to reduce unintended pregnancies and abortion rates. This study has demonstrated that having access to no-cost contraception helps us get to that goal," said Alina Salganicoff, director of women's health policy at the Kaiser Family Foundation.
The law requires that Food and Drug Administration-approved contraceptives be available for free for women enrolled in most workplace insurance plans, a change that many will see as new plan years begin Jan. 1.
This policy is among the law's most contentious provisions because it exempts churches that oppose contraception but requires religious-affiliated organizations, such as colleges or hospitals, to provide the coverage for their workers.
The U.S. Conference of Catholic Bishops and many conservative groups say this violates religious freedom, and Republican presidential nominee Mitt Romney has voiced similar criticism.
About half of unplanned pregnancies occur in women who do not use contraception. For the other half, condoms can fail and so can birth control pills or other shorter-acting methods if the woman forgets to use them or can't afford a refill.
On the other hand, an IUD, a tiny T-shaped device inserted into the uterus, can last for five to 10 years, depending on the brand.
Peipert said only about 5 percent of U.S. women use long-acting contraceptives, which is far fewer than in other developed countries. Insurance doesn't always cover the high upfront cost to insert the IUD, even though years of birth control pills can add up to the same price, said Peipert.
Cost isn't the only issue. Doctors don't always mention long-acting methods, maybe because of a long-outdated belief that IUDs aren't for young women or because they assume women want the pill, which is a more commonly prescribed.
The Associated Press contributed to this report.