CHICAGO (AP) — Babies born even just a few weeks prematurely run a significantly higher risk of death in their first year, suggesting that inducing early labor is more dangerous than many obstetricians might think, researchers say.

A normal pregnancy lasts about 40 weeks, and babies born before 37 weeks are considered premature. The risks faced by very premature babies — those born before 32 weeks — are well-documented.

But over the past decade, doctors have begun using drugs more and more to induce labor a few weeks early for reasons of convenience rather than the health of the mother or child.

The new study on the risks of slightly premature births was published in Wednesday's Journal of the American Medical Association.

It was led by Dr. Michael Kramer of McGill University in Montreal.

Based on an examination of 4.5 million births in the United States and Canada in the 1990s, the researchers found that compared with U.S. babies born full-term in 1995, those born at 32 weeks to 33 weeks were about six times more likely to die within their first year.

Babies born closer to term but still early — at 34 through 36 weeks — were nearly three times more likely to die than full-term infants.

The causes of death included infection, breathing problems, various birth defects and Sudden Infant Death Syndrome.

Obstetricians "may perceive the induction as risk-free and therefore not adequately balance the risks and benefits," Kramer said.

Most of the estimated 250,000 U.S. infants delivered prematurely each year are born after the 32nd week. With medical advances in the past decade, about 98 percent of these slightly premature babies survive.

Dr. Dan Polk, a neonatologist at Children's Memorial Hospital in Chicago, said his wife's obstetrician wanted to induce labor early because he feared she would need a Caesarean section and he wanted to avoid having to come to the hospital at night to perform the emergency surgery. Ultimately, she underwent a C-section on her due date.

Though that doctor wanted to induce labor at 38 weeks — early but not technically prematurely — it is not uncommon for others to induce labor even earlier — at 35 or 36 weeks, for example — for the convenience of the mother or the doctor, Polk said.

Polk said that the advent of medical treatment for inadequately functioning lungs — a common complication in premature infants — has created the misperception that labor can be induced early with few risks.

The lung treatment "doesn't do anything for their kidneys, their hearts or skin, all of which have problems involved in preterm births," Polk said.

Jennifer L. Howse, president of the March of Dimes, said the study shows that "every day of development during pregnancy is precious to the health of an unborn baby."

The study "gives even greater urgency to our efforts to find ways to prevent preterm birth from occurring," Howse said.

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