WEDNESDAY, Jan. 7 (HealthDay News) -- Women having an elective repeat C-section should wait until they're at least 39 weeks into their pregnancy to have the baby, if there are no medical issues with the mother or baby, a new study found.

Delivering just two weeks earlier doubles the risk that the baby will have problems, such as trouble breathing, infection and low blood sugar, according to the study.

The findings buttress recommendations from the American College of Obstetricians and Gynecologists (ACOG).

"About 36 percent of women were delivered prior to 39 weeks, electively by Caesarean," said the study's lead author, Dr. Alan T.N. Tita, an assistant professor of obstetrics and gynecology and an epidemiologist at the University of Alabama at Birmingham. "These early deliveries were associated with adverse outcomes. There was a two-fold increase in morbidity in those delivered at 37 weeks compared to women delivered at 39 weeks."

Results of the study are published in the Jan. 8 issue of the New England Journal of Medicine.

Once a woman has had a Caesarean delivery, she and her doctor may decide to perform an "elective" C-section for subsequent births. The recommendation is that any such delivery should occur at 39 weeks or later if there are no medical concerns for the mother or baby. If a woman wants to deliver before 39 weeks, ACOG recommends that amniocentesis be performed to assess lung maturity in the baby.

The new study included 13,258 women who underwent elective repeat Caesarean deliveries at one of 19 U.S. academic medical centers. All of the sites are part of the Eunice Kennedy Shriver National Institute of Child Health and Human Development's Maternal-Fetal Medicine Units Network.

Of the elective C-sections, 35.8 percent were performed before 39 weeks of gestation, with 6.3 percent done at 37 weeks and 29.5 percent at 38 weeks, the study found.

Babies born at 37 weeks had twice the rate of certain complications, such as needing to be placed on a mechanical ventilator, newborn sepsis (a blood infection) and low blood sugar. Babies delivered at 38 weeks had a 1.5 times higher rate of complications, according to the study.

Dr. Peter Bernstein, a maternal-fetal medicine specialist at Montefiore Medical Center and Albert Einstein College of Medicine in New York City, said he was not surprised by the increase in complications for deliveries before 39 weeks. "The bigger surprise to me was that nearly 36 percent of elective procedures were done before the 39th week," Bernstein said.

"The institutions that participate in this network are big academic centers," he said. "In the academic world, these are among the top institutions, and that more than one-third apparently aren't following ACOG guidelines is a surprise."

Both Tita and Bernstein said that early deliveries are probably due to both mothers and doctors. "There's an interplay between patient desires and physician willingness to accommodate patients," Tita said.

Bernstein said that women often prefer their own doctor for the delivery and not another doctor in the practice, which can lead to scheduled deliveries. And, he added, when you try to accommodate both the mother's and the doctor's schedule, it's easy to see how things get pushed back a week or more.

But he added a caution. "We have to not fool around with this," he said. "The baby may not be ready. This study's findings underline that the ACOG guidelines are appropriate."

Tita added, "In the absence of complications, it's probably optimal to wait until 39 weeks to undergo elective Caesarean."

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