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Typical Pregnancy Now 39 Weeks, Not 40

March 23, 2006 -- The most common length of a pregnancy in the U.S. is a week shorter than it was just over a decade ago, dropping from 40 weeks to 39 weeks, according to a new analysis from the March of Dimes.

A dramatic rise in late preterm births, or babies born between 34 and 36 weeks gestation, was also seen between 1992 and 2002. Researchers reported a 12% rise in these late preterm births during the 10-year study period, representing almost three-quarters of all preterm births.

Better fetal monitoring, which has led to an increase in medically assisted births, is largely responsible for the increase. The practice has led to a decline in stillbirths, delivery-related deaths, and serious delivery-related medical problems.

But March of Dimes medical director Nancy Green, MD, tells WebMD there is a concern that at least some of the rise in assisted early births is not medically justified.

She points out that babies born even a few weeks early often need more medical care, and they are at greater risk for respiratory and feeding difficulties, as well as jaundice, reduced brain development, and problems regulating temperature.

"Late preterm infants are a growing concern," she says. "We would like to see more medically uncomplicated births go to term."

Preterm Patterns

Babies are considered preterm if they are born prior to 37 completed weeks of gestation. Carrying two or more babies is a big risk factor for early delivery, but the March of Dimes analysis included only single births.

Researcher Michael J. Davidoff, MPH, tells WebMD that increasing rates of cesarean section deliveries and induced labor do not fully explain the rise in late preterm births. Similar increases in the late preterm birth rate were seen among women who had these medical procedures and those who did not.

Other factors, such as the rise in maternal obesity and age, may contribute to early births, Green and Davidoff said.

"More and more women who give birth are overweight or obese," Green says. "These women have a much higher rate of complications like diabetes and hypertension, which can lead to earlier births."

Of the roughly 4 million deliveries in the U.S. in 2002, 394,996 were considered preterm, according to the analysis. Late preterm deliveries accounted for three-fourths of these early deliveries, between 34 and 36 completed weeks.

The incidence of very early deliveries, prior to week 34, has remained relatively stable over the last several decades, Green says.

Doctors May Not See Risks

Epidemiologist David Savitz, PhD, who has long studied preterm delivery trends, says it makes sense that better fetal monitoring has led to earlier deliveries, especially since the risks involved are not readily evident at the clinical level.

Savitz is a professor of community and preventive medicine at Mount Sinai Medical Center in New York.

"When you look at large populations there are small but very real increases in the risk of adverse outcomes for those 34-, 35- and 36-week babies, but it may be something that an individual clinician never sees," he says. "If there is a major problem being prevented then early delivery is absolutely justified. But it is important for both the clinician and patient to be aware that this risk exists."

In the March of Dimes report, published March 23 in a supplemental issue of the journal Seminars in Perinatology, the March of Dimes researchers called for better studies of outcomes among late preterm babies.

"Excess neonatal complications among these infants, even at 35 to 36 weeks gestation, may require a re-assessment of optimal obstetric and neonatal care," they wrote.