What if you could make the difference between life and death for your baby, simply by being patient? A new study published in the June issue of Obstetrics & Gynecology shows that mortality rates are halved by waiting until at least 39 weeks rather than 37 weeks to give birth.

The study is the largest to confirm a message that public-health agencies and professional medical groups have been eager to spread: early elective deliveries are a bad idea. “Up until the last several years, we thought term pregnancies between 37 and 41 weeks were the same,” says Alan Fleischman, medical director at the March of Dimes. “This is not the case. It’s a biological continuum. The new data makes us pause and realize we ought not intervene unless there’s a very good medical reason.”

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Researchers at the National Institutes of Health, the March of Dimes and the U.S. Food and Drug Administration collaborated to analyze mortality rates for babies born between 37 and 40 weeks. Forty weeks is an actual full-term pregnancy, but many have considered gestation to weeks 37 to 38 as more or less equivalent; babies born before 37 weeks are classified as pre-term.

Yet the researchers found that babies born at 37 weeks had twice the risk of death as 40-weekers, regardless of race or ethnicity. Using 2006 statistics, the team found that the infant mortality rate was 3.9 per 1,000 babies born at 37 weeks, compared with 1.9 deaths for every 1,000 live births for babies born at 40 weeks. “Mortality is the tip of an iceberg so there are large numbers of babies who don’t die but are sick and require neonatal intensive care interventions and hospitalizations that they would not have needed if they were born a few weeks later,” says Fleischman.

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Of course, there are situations in which early delivery is essential for medical reasons. But the new research highlights the importance of not scheduling delivery electively before 39 weeks at the earliest, which dovetails with a recommendation from the American College of Obstetricians and Gynecologists. Earlier this year, the March of Dimes called on hospitals to actively combat the surge in early elective deliveries by requiring proof of medical necessity from doctors scheduling such procedures. In explaining the rationale for early delivery, I wrote:

On one hand, it’s understandable. Doctors want to be able to better control their schedule, eliminating middle-of-the-night deliveries and ensuring that they — and not one of their partners — delivers a baby since the delivering physician often receives the bulk of reimbursement. … It’s kind of surprising that insurance providers haven’t curtailed the practice of early elective deliveries entirely as babies born sooner tend to have more health complications and cost more. Even babies delivered at 37 to 38 weeks can end up costing 10 times as much as a full-term newborn, according to the March of Dimes. One study found that reducing early elective delivies to under 2% could save close to $1 billion in health care each year.

Although not dangerously premature, babies born around the 37-week mark are more likely to have breathing problems and require ventilation. They may have higher bilirubin levels, which contribute to jaundice, or low blood sugar.

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Often, those charged with taking care of babies are to blame for too-early deliveries. “There’s collusion between mothers and doctors,” says Fleischman. “Sometimes the mother is suggesting early delivery and sometimes it’s the doctor.”