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Typical U.S. Pregnancy Now Just 39 Weeks

That's a week shorter than the traditional full-term pregnancy, report finds

THURSDAY, March 23, 2006 (HealthDay News) -- The average length of a pregnancy in the United States is getting shorter, with the most common duration now 39 weeks, rather than 40 weeks, which is considered full term, a new report found.

Babies born close to full term -- five to six weeks early, called "late preterm" -- now account for the lion's share of premature births in the United States. And those babies, like very premature babies, face greater health risks.

"Most of the rise in prematurity is related to these so-called late preterm babies," said Dr. Nancy Green, medical director of the March of Dimes in White Plains, N.Y. "In terms of numbers, it's enormous," she said.

About one in eight babies is born premature and 70 percent of those are late preterm, accounting for more than 355,000 births a year, said Michael Davidoff, manager of informatics, research and development at the March of Dimes and lead author of the paper, which appears in a special supplement of the journal Seminars in Perinatology. The paper was also presented at a July 2005 symposium sponsored by the National Institute of Child Health and Human Development.

The proportion of all preterm births in the United States has been rising steadily, from 9.4 percent of live births in 1981 to 12.3 percent in 2003, a 31 percent increase. Healthy People 2010, a federal health initiative, has established a target rate of no more than 7.6 percent.

But, during this period, the rate of very preterm births has stayed relatively constant at 1.8 percent to 2 percent. These very-early babies have been the focus of much attention in the past, because they have high complication rates. Prematurity and low birthweight are the leading cause of infant mortality among black infants and the second leading cause of mortality among all infants.

Attention is now shifting to the "later preterm" babies because they, too, can suffer complications such as respiratory distress syndrome, hypoglycemia and longer stays in the neonatal intensive care unit (ICU).

"They certainly have more complications at birth and end up anywhere from a few days to a few weeks in the neonatal ICU," Green said.

"Nobody has been focusing on the babies at more than 34 weeks -- are we doing the right thing by not bringing them to full-term ?" added Dr. Amanda Cotter, assistant professor of obstetrics and gynecology at the University of Miami Miller School of Medicine.

And although the impact on a single infant may not be huge, the public health ramifications are considerable. "The total hospital cost for these infants is about the same as it is for the extremely preterm babies because there are fewer of the extremely preterm infants," Green said.

According to the new report, 25 percent of all singleton babies were born full term at 39 weeks in 2002. Between 1992 and 2002, births at or after 40 weeks declined by nearly 21 percent while births occurring between 34 and 36 weeks increased 12 percent.


The study authors speculated that the rise of Caesarean sections and induced labor, as women become more focused on controlling their delivery dates, may be a part of it. But that is far from the whole picture, Green said, adding, "There's no hard data."

And early delivery isn't always a bad thing.

"Some of this is a success story, better maternal and fetal monitoring, detecting problems early and intervening before calamities happen," Green said. "We know that mortality rates have gone down over time, which largely reflects better management," she added.

On the other hand, she continued, "Some of the late preterm babies who have complications and who spend a few days in the neonatal ICU, we suspect, [the early delivery] may not be medically necessary."

What should expectant parents do?

Make your pregnancy as healthy as can be, starting, if possible, before conception with proper nutrition and lifestyle habits, Green advised. Detect and treat underlying maternal health conditions whether it's hypertension, asthma or diabetes.

Finally, Green said, "if everything is going well for a singleton pregnancy, and everything is uncomplicated, go to term. Don't deliver early, because there are consequences."

More information

For more on a healthy pregnancy, visit the March of Dimes.

SOURCES: Nancy Green, M.D., medical director, March of Dimes, White Plains, N.Y.; Michael J. Davidoff, manager of informatics, research and development, March of Dimes, White Plains, N.Y.; Amanda Cotter, M.D., assistant professor, obstetrics and gynecology, University of Miami School of Medicine; March 2006, Seminars in Perinatology
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