WALSENBURG, Colo. — Over the past six years, Colorado has conducted one of the largest ever real-life experiments with long-acting birth control. If teenagers and poor women were offered free intrauterine devices and implants that prevent pregnancy for years, state officials asked, would those women choose them?

They did in a big way, and the results were startling. The birthrate for teenagers across the state plunged by 40 percent from 2009 to 2013, while their rate of abortions fell by 42 percent, according to the Colorado Department of Public Health and Environment. There was a similar decline in births for another group particularly vulnerable to unplanned pregnancies: unmarried women under 25 who have not finished high school.

“Our demographer came into my office with a chart and said, ‘Greta, look at this, we’ve never seen this before,’ ” said Greta Klingler, the family planning supervisor for the public health department. “The numbers were plummeting.”

The changes were particularly pronounced in the poorest areas of the state, places like Walsenburg, a small city in Southern Colorado where jobs are scarce and unplanned births come often to the young. Hope Martinez, a 20-year-old nursing home receptionist here, recently had a small rod implanted under the skin of her upper arm to prevent pregnancy for three years. She has big plans — to marry, to move West, and to become a dental hygienist.

“I don’t want any babies for a while,” she said.

More young women are making that choice. In 2009, half of all first births to women in the poorest areas of the state happened before they turned 21. By 2014, half of first births did not occur until they had turned 24, a difference that advocates say gives young women time to finish their educations and to gain a foothold in an increasingly competitive job market.

“If we want to reduce poverty, one of the simplest, fastest and cheapest things we could do would be to make sure that as few people as possible become parents before they actually want to,” said Isabel Sawhill, an economist at the Brookings Institution. She argues in her 2014 book, “Generation Unbound: Drifting Into Sex and Parenthood Without Marriage,” that single parenthood is a principal driver of inequality and long-acting birth control a powerful tool to prevent it.

Teenage births have been declining nationally, but experts say the timing and magnitude of the reductions in Colorado are a strong indication that the state’s program was a major driver. About one-fifth of women ages 18 to 44 in Colorado now use a long-acting method, a substantial increase driven largely by teenagers and poor women.

The surge in Colorado has far outpaced the growing use of such methods nationwide. About 7 percent of American women ages 15 to 44 used long-acting birth control from 2011 to 2013, the most recent period studied, up from 1.5 percent in 2002. The figures include all women, even those who were pregnant or sterilized. The share of long-acting contraception users among just women using birth control is likely to be higher.

But the experiment in Colorado is entering an uncertain new phase that will test a central promise of the Affordable Care Act: free contraception.

The private grant that funds the state program has started to run out, and while many young women are expected to be covered under the health care law, some plans have required payment or offered only certain methods, problems the Obama administration is trying to correct. What is more, only new plans are required to provide free contraception, so women on plans that predate the law may not qualify. (In 2014, about a quarter of people covered through their employers were on grandfathered plans, according to the Kaiser Family Foundation.) Advocates also worry that teenagers — who can currently get the devices at clinics confidentially — may be less likely to get the devices through their parents’ insurance. Long-acting devices can cost between $800 and $900.

“There’s no lifeboat with the Affordable Care Act,” said Liz Romer, a nurse practitioner who runs the Adolescent Family Planning Clinic at Children’s Hospital Colorado, which went from giving out 30 long-acting devices a year in 2009 to more than 2,000 in 2013.

The state failed to get additional funding through the General Assembly this spring, a shortfall Ms. Klingler said would slow, but not stop, the state’s progress.

Women’s health advocates contend that long-acting birth control is giving American women more say over when — and with whom — they have children. About half of the 6.6 million pregnancies a year in the United States are unintended. Teenage births may be down, but unplanned births have simply moved up the age scale, Ms. Sawhill said, and having a baby before finishing college can be just as risky to a woman’s future as having one while in high school.

Colorado’s program, funded by a private grant from the Susan Thompson Buffett Foundation, named for the billionaire investor Warren Buffett’s late wife, was the real-world version of a research study in St. Louis, (also paid for by the foundation, which does not publicly acknowledge its role) that came to the same conclusion: Women overwhelmingly chose the long-acting methods, and pregnancy and abortion rates plunged.

“The difference in effectiveness is profound,” said Dr. Jeffrey Peipert, a professor of obstetrics and gynecology at Washington University in St. Louis, who ran the study. The failure rate for the pill was about 5 percent, compared with less than 1 percent for implants and IUDs.

The methods are effective because, unlike the pill, a diaphragm or condoms, they do not require a woman to take action to work. And while an early incarnation, the Dalkon Shield introduced in the 1970s, had disastrous results, the modern devices are safe and have been increasingly promoted by doctors. Last fall, the American Academy of Pediatrics published guidelines that for the first time singled them out as a “first-line” birth control option for adolescents, citing their “efficacy, safety and ease of use.”

“There’s been a big shift in the mind-set,” said Dr. Laura MacIsaac, director of family planning for Mount Sinai Beth Israel in New York. “The demand is coming from everywhere now.”

In Walsenburg, Colo., studded with boarded-up buildings and weedy parking lots, advocates have used the Buffett grant to help women get more control. Poverty erodes health here: Last year, rural Huerfano County, which includes Walsenburg, was ranked second-to-last for life expectancy in the state.

“If you get pregnant here, you are stuck,” said Debbie Channel, the manager of the Spanish Peaks Regional Health Center’s Outreach and Women’s Clinic, where Ms. Martinez, the nursing home receptionist, got her implant. “We’re trying to keep them safe and baby free.”

Proponents say the program is working. The state health department estimated that every dollar spent on the long-acting birth control initiative saved $5.85 for the state’s Medicaid program, which covers more than three-quarters of teenage pregnancies and births. Enrollment in the federal nutrition program for women with young children declined by nearly a quarter between 2010 and 2013.

Ms. Martinez had a preview of life as a mother while babysitting two young stepbrothers. She found it exhausting. She watched girls in her high school get trapped when they became mothers too soon, and she knew she did not want that for herself.

“They say they want to leave Walsenburg, but they never do,” Ms. Martinez said. “It’s a circle. It keeps happening and happening.”

She plans to leave town this summer after she marries her fiancé, now in California in military training. She is excited about the next chapter.

“I’m not scared at all,” she said. “I’m just really ready.”