Complicated births can be scary for pregnant women, who often have no choice but to go into labor a month or more early. But a new study from the Children's Hospital of Philadelphia finds that, all too often, women and their doctors choose to deliver healthy babies two or three weeks early.

To get a better idea of how often pregnant women and their doctors go ahead with an elective early C-section or induction, the team used 15 years of birth certificate and discharge data in Pennsylvania, Missouri and California.

Publishing in the July issue of Medical Care, CHOP neonatologist Scott Lorch said they found that about 3 to 4 percent — or nearly one in 25 babies — were delivered early for no medical reason.

"That may not sound like a humongous number," said Lorch. "But we need to take this in context of there's about 4 million births a year in the U.S. So even the number of 3.5 to 4 percent is anywhere between 150,000 to160,000 deliveries a year."

While unnecessary early births appear to be on the decline from the 4 percent peak in 2005, Lorch said it's unlikely to have decreased to the low of 2.1 percent in 1995.

Babies born a few weeks early aren't considered premature and are usually healthy. But Lorch said delivering even slightly early can have consequences.

"The risk of, for example, an early term, non-indicated cesarean delivery of having respiratory problems after birth was almost 2.5 times higher than it was in babies who had a term delivery or even a spontaneous delivery at the same gestational age," said Lorch.

Elective early C-sections and inductions were both associated with babies having to stay longer in the hospital, often from difficulties in feeding.

It's not entirely clear why so many babies are delivered early, but older, more educated and privately insured women were most likely to do so. Access to care may also play a role, as women giving birth in smaller hospitals were more likely to opt for delivering early.

The best course, Lorch said, is to let the baby decide when it's time to be born.