This complements a CDC report in January that found the first-time cesarean section rate also declined during that three-year period. Women whose labor is induced are more likely to wind up having a cesarean, and women who have a first cesarean are almost sure to have surgery for subsequent deliveries.
To be sure, the declines are tiny, and the rates are still high. The 2012 induction rate was 23.3 percent, compared with less than 10 percent in 1990. The 2012 first-time cesarean rate was 21.5 percent, compared with 15.5 percent in 1999.
Still, experts are heartened. Research shows that most premature deliveries - before 39 weeks - are not medically necessary. Studies also show newborns who lose even a few weeks in the womb are more likely to have problems with feeding, breathing, and staying warm, and to need extra time in the hospital.
"Although the declines are very modest, the news is encouraging," CDC officials said.
What explains these mini-trends?
At the Birth Center, a midwifery practice in Bryn Mawr, clinical services director Julie E. Cristol credited the March of Dimes' decade-old marketing campaign, "Healthy Babies are Worth the Wait."
"We owe it all to the March of Dimes," she said. "They gave out buttons that said '39 Weeks.' They did an amazing job of raising awareness at the consumer level."
Credit also goes to midwives, who have long decried the "medicalization" of childbirth, and eschew interventions including labor induction, episiotomies, and cesareans.
At Drexel University College of Medicine, midwives are key partners in maternity care, said Owen Montgomery, chair of obstetrics and gynecology.
"I think the places that are reducing inductions and C-sections are utilizing all this new knowledge, and being a little more hands-off," Montgomery said. "They're behaving a little more like midwives. And that's a good thing."
Many cesareans occur because of a catchall problem doctors call "failure to progress," which means, in essence, that labor is moving too slowly.
But recent research has found that labor generally takes longer now than 50 years ago, reflecting factors such as maternal obesity and age - and labor induction.
In March, two influential groups - the American College of Obstetricians and Gynecologists and the Society for Maternal-Fetal Medicine - issued new guidelines that cited this change in labor patterns and urged greater patience with patients.
Will the trends continue?
"I've been in practice since the 1980s and I still don't really know what makes the pendulum swing, or not swing," said Cristol, the midwife. "In the 1980s, the feminist movement made women question things. Now, again, women are questioning. So we'll see."