About one in every 25 U.S. babies are born earlier than is "medically justified" through processes such as elective C-sections or induced labor.

The study reinforces the importance of abstaining from early-term deliveries without adequate medical reasons behind them, a Children's Hospital of Philadelphia news release reported.

"A growing body of research suggests that health outcomes are worse for infants born before 40 weeks gestation, compared to full-term births," Scott A. Lorch, M.D., M.S.C.E., a neonatologist at The Children's Hospital of Philadelphia (CHOP), said in the news release. "Unfortunately, many of these earlier births are 'nonindicated,' meaning there is not a medical rationale to deliver the baby early. We analyzed the extent to which these infants are born too soon and without medical indication."

"Our study showed that early elective deliveries are making up between 3 and 4 percent of U.S. births each year," Katy B. Kozhimannil, Ph.D., MPA, of the University of Minnesota School of Public Health, said in the news release.  "This may seem to be a small number, but with 4 million births a year in the U.S., each percentage point represents 40,000 babies."

The researchers looked at 7.3 million uncomplicated term births that took place over the course of 15 years in California, Missouri, and Pennsylvania. The early-term nonindicated birth rate was at 3.18 percent over the full study period. The rate in peaked in 2006; at this time four percent of uncomplicated births occurred before 39 weeks of gestation without medical indication. By 2009 that rate dropped to 3.74 percent, which is 86 percent higher than it was in 1995.

"Our study team found that nonindicated early births had adverse consequences for newborns and families," Lorch said.

Early-term cesarean sections can double the baby's change of experiencing respiratory distress or requiring ventilation. Both early-term non-indicated cesarean sections and induces labor were found to lengthen the newborn's hospital stay.

"While prior research has shown that early elective delivery policies can be highly effective within particular healthcare systems, there is a need to address this issue at the population level," Kozhimannil said. "It is our hope that this study will add fuel to the ongoing efforts to educate pregnant women and influence clinical and policy environments to facilitate healthy, full-term deliveries whenever possible."