“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 the women 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 is 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.

Image An intrauterine device, which prevents pregnancy for several years, at a clinic in Walsenburg, Colo. A state program that provides long-acting birth control has contributed to a sharp decline in birth and abortion rates among teenagers. Credit... Benjamin Rasmussen for The New York Times

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.