FRIDAY, Nov. 20 (HealthDay News) -- Recent reports show that the rate of preterm deliveries continues to climb in the United States. Now, a new study suggests one reason why: Many women are confused about what constitutes a full-term birth in the first place.

About one-quarter of new mothers surveyed in the study considered a baby born at 34 to 36 weeks of gestation to be full term, while slightly more than half of women considered 37 to 38 weeks full term.

Though technically speaking, preterm births are babies born prior to 37 weeks, 39 to 40 weeks is optimal, according to the researchers.

Many women interviewed were also unaware that babies born even a little bit premature are at a higher risk of serious health problems compared to babies born at term, the new survey shows.

Misconceptions about what constitutes full gestation and how soon it's safe to schedule an elective induction or cesarean delivery are contributing to increasing numbers of premature births in the United States, said lead study author Dr. Robert L. Goldenberg, professor of obstetrics and director of research at Drexel University College of Medicine in Philadelphia.

"Clearly, the preterm birth rate is going up, as are early deliveries that are at term but are 37 and 38 weeks," Goldenberg said. "The data is becoming more and more clear that the outcomes of births at those earlier gestational ages are not as good as babies that are born at 39 or 40 weeks."

The study, which included 650 first-time mothers ages 21 to 45 from around the nation who had health insurance, is in the December issue of Obstetrics & Gynecology.

When asked, "What is the earliest point in pregnancy that it is safe to deliver the baby, should there not be other medical complications requiring early delivery?", more than half chose 34 to 36 weeks, 41 percent chose 37 to 38 weeks and less than 8 percent chose 39 to 40 weeks.

However, experts warn that any delivery short of 39 weeks puts a baby at higher risk of respiratory distress, sepsis (blood infection) and needing to be placed in the neonatal intensive care unit, according to background information in the study. Only one-quarter of new moms realized 39 to 40 weeks was safest.

Premature births are a growing problem in the United States. In fact, the percentage of babies born preterm rose by more than 20 percent from 1990 to 2006, according to a report released in November by the U.S. National Center for Health Statistics.

Technically, the World Health Organization and other major medical organizations define preterm births as babies born before 37 weeks. But that definition was developed some 50 years ago and is outdated, said Dr. Alan Fleischman, medical director for the March of Dimes.

More recently, studies have shown that babies born even a bit too early -- at 37 or 38 weeks -- have a greater chance of chronic respiratory disease and learning disorders than children born at 39 weeks or later.

Babies born between 34 and 37 weeks are six times more likely to die during their first week or life and three times more likely to die during their first year than babies born at 39 or 40 weeks, Fleishman added.

"Everybody knows a baby who has been born a bit early who has done pretty well," Fleischman said. "But what we've learned is that, going backwards, there is increasing mortality and morbidity for every week prior to 39 weeks of gestation."

Many experts now refer to babies born between 34 and 36 weeks as "late preterm," while babies born at 37 and 38 weeks are "early term."

The American College of Obstetricians and Gynecologists and the March of Dimes recommend against elective inductions or C-sections prior to 39 weeks.

In many situations, there is probably some medical reason for choosing to deliver early -- perhaps the mother has slightly elevated blood pressure, for example, Goldenberg said.

"I call them semi-electives," Goldenberg said. "I believe over the last 15 or 20 years, the practice is evolving to deliver those babies earlier and earlier when there is no evidence of benefits."

TV shows and news reports about very premature babies that survive may also be fueling misconceptions, Goldenberg said. Some women are left with the impression that if babies born before 30 weeks can survive, infants that are just a little bit premature should have no problems.

"Because the shows don't emphasize the bad outcomes at those ages, it's led not only women but doctors to conclude that by the time you get up to 34, 35 or 36 weeks, everything is fine," Goldenberg said. "But the recent research is showing it's not fine."

The last few weeks of gestation are critical to fetal development. All of the organs continue to mature in preparation for moving from the womb to the outside world, Fleischman explained. Between 35 and 40 weeks, the fetal brain grows by about 50 percent.

Educating expectant mothers and their physicians about the risk of preterm births may help women to make more informed decisions about when to schedule elective inductions and C-sections, Goldenberg said. That includes setting up hospital policies that discourage elective deliveries prior to 39 weeks and enforcing it through peer review to help curb the practice.

More information

There's more on preventing premature births at the March of Dimes.

SOURCES: Robert L. Goldenberg, M.D., professor of obstetrics and director of research, Drexel University College of Medicine in Philadelphia, Pa.; Alan Fleischman, M.D., medical director, March of Dimes; Obstetrics & Gynecology, December, 2009.

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