Colorado's Department of Public Health and Environment is seeking more funding to continue a privately funded birth control program that has, by several measures, been a startling success.

The program, known as the Colorado Family Planning Initiative, provides intrauterine devices (IUDs) or implants at little to no cost for low-income women at family planning clinics in Colorado. It contributed to a 40 percent drop in Colorado's teen birth rate and a 42 percent drop in the state's teen abortion rate between 2009 and 2013, according to state data reported by the New York Times's Sabrina Tavernise.

Related The mystery of the falling teen birth rate

Young women served by the family planning clinics also accounted for about three-fourths of the overall decline in Colorado's teen birth rate. And the infant caseload for Colorado WIC, a nutrition program for low-income women and their babies, fell by 23 percent from 2008 to 2013.

"This initiative has saved Colorado millions of dollars," Gov. John Hickenlooper said in a July 2014 statement. "But more importantly, it has helped thousands of young Colorado women continue their education, pursue their professional goals and postpone pregnancy until they are ready to start a family."

But the program has long drawn criticisms from social conservatives, who argue that it could encourage promiscuity. Since teens don't need to be accompanied by an adult to obtain contraceptives at the facilities, critics also say the initiative undermines parental rights. And some opponents reject the states' numbers altogether. Lawmakers in Colorado's Republican-controlled Senate made similar arguments when they blocked public funding for the program earlier this year.

Colorado's drop in teen births is part of a nationwide decline in the teen birth rate. There are multiple theories for this decline, ranging from greater use of long-acting reversible contraceptives to lead abatement programs.

Still, Colorado's teen birth rate seems to be declining much more quickly than its peers' rates, as this chart from University of Maryland sociologist Philip Cohen shows:

Between 2008 and 2012, the state went from the 29th-lowest teen birth rate in the nation to the 19th-lowest. And while one-fifth of women ages 18 to 44 in Colorado now use a long-acting birth control method, about 7 percent of US women in the same age group did from 2011 to 2013, according to Centers for Disease Control and Prevention data.

But the private grant that has sustained the program is starting to run out — and state lawmakers don't seem interested in filling the gap with public funds, leaving its future uncertain.