Three in 10 girls and women in the United States become pregnant before 20, a rate significantly higher than that in many other rich countries. The 14-to-19-year-old participants in the study Ms. House joined, nearly half of whom had already had an unintended pregnancy, were offered free birth control and counseled on the benefits of long-acting contraceptives like intrauterine devices and implants, methods used by fewer than 5 percent of teenagers.

Overwhelmingly, Ms. House and other teenagers — 72 percent of the 1,404 participants — chose long-acting birth control. And it had an enormous effect.

Pregnancy and abortion rates plunged to less than a quarter the rates of sexually experienced teenagers nationally, the group most comparable to those in the study. Rates were also significantly lower than those among all teenagers.