Dec. 11, 2007 -- Babies born by elective C-section before the 39th week of pregnancy have a three- to fourfold higher risk of breathing trouble than babies whose mothers have a normal vaginal delivery.
Elective C-section babies also have a fivefold higher risk of needing mechanical breathing assistance for serious respiratory trouble, find Anne Kirkeby Hansen, MD, and colleagues at Denmark's Aarhus University Hospital.
"Mothers who choose elective cesarean section should be aware that the risk of respiratory problems is four times raised at 37 weeks' gestation vs. full-term, intended vaginal delivery," Kirkeby Hansen tells WebMD. "The rate of respiratory problems is 10% for elective C-section at 37 weeks, but it is 2.8% for intended vaginal deliveries. That is why we say you should never do elective cesarean section at 37 weeks."
Kirkeby Hansen and colleagues gathered data on the 34,458 babies born in Aarhus, Denmark, from 1998 through 2006. Nearly 2,700 of these infants were delivered via elective C-section -- that is, the mother or her obstetrician opted for C-section without having a medical need to so.
The researchers compared these infants to infants from women who tried to have a vaginal delivery, including women who ended up having a C-section.
After adjusting for factors that might affect the infant's breathing, Kirkeby Hansen and colleagues found that children delivered by elective C-section at 37 weeks' gestation had a 3.7-fold higher risk -- and at 38 weeks, a 3.0-fold higher risk -- of transitory tachypnea of the newborn (a condition sometimes called wet lung), respiratory distress syndrome, or persistent pulmonary hypertension (dangerously high blood pressure in the lungs).
All of these conditions mean that a baby is placed in an incubator in the neonatal intensive care unit for two days or so, Kirkeby Hansen says.
Most children fully recover from these breathing problems, notes Emory University pediatrician Lucky Jain, MD. But the long-terms effects aren't clear.
"Sometimes these babies get into bigger trouble in the neonatal ICU," Jain tells WebMD. "And what we don't yet understand well is the impact of two or three or four days of separation from the mother, of not initiating breastfeeding, and of exposure to bacteria that are not normally found in our bodies."
Although it happened much less often, the Danish researchers found that children delivered via elective C-section at 37 weeks' gestation have a fivefold higher risk of serious breathing problems requiring oxygen therapy, a continuous positive air pressure device, or mechanical ventilation. For elective C-sections at 38 weeks' gestation, this risk is 4.2 times higher than for intended full-term vaginal delivery.
Labor Good for Fetus
What does a C-section have to do with a newborn's ability to breathe?
As it leaves the liquid environment of the womb, a newborn faces the enormous challenge of making the transition to breathing air. Its fluid-filled lungs must clear quickly, Jain notes.
"There are many reasons why a baby born after elective C-section is more prone to delayed transition to air breathing," Jain says. "The first is reduced gestational age. And in the last trimester of pregnancy, every week counts. A 37-weeker is much more prone to respiratory issues than a 39-weeker."
Kirkeby Hansen and Jain note that during labor, a woman secretes powerful stress hormones. This triggers stress-hormone secretion in her fetus. The hormones have two effects on the fetal lungs. They speed the absorption of liquid. And they increase secretion of surfactants, natural substances that help clear liquid from the lungs.
"Once a woman is in labor, all this gets started," Kirkeby Hansen says. "In women who do not have labor, this process is not believed to start."
Jain says labor is the most reliable sign that a baby is ready to be born.
"When mother nature calls on spontaneous labor to start, it mostly is accurate in terms of the biologic clock and a good likelihood the baby is mature," he says. "But when we do it by elective C-section, we trust mothers' last-period dates or ultrasounds performed early in pregnancy, and those calculations are not always accurate."
Nearly a third of U.S. pregnancies now end in C-sections, Jain says. Over the last decade, as the C-section rate has climbed, the average gestational age at birth for U.S. babies has dropped from 40 weeks to 39 weeks.
On the one hand, Jain notes, research shows that delivering infants at 39 weeks' gestation or less cuts the risk of stillbirths. On the other hand, early delivery clearly has its own risks -- to the infant as well as to the mother.
"The obstetric community has to get its arms around the fact that C-section has never been proven to be safer for the mother," Jain says. "A study that appeared last year showed that when you look at mothers with no identified risk who have had a C-section -- with no medical indication either from the mother or from the fetus -- there was higher mortality in the mother and in the baby."
Kirkeby Hansen advises women seeking elective C-section to wait until the 39th week of their pregnancy.
"A woman should make sure she is not having her C-section too early. She should put her foot down and not have it at 37 or 38 weeks just because this fits into the hospital's plan," she says. "I personally would not have one before 39 weeks."
"Thirty-nine weeks' gestation is a minimum. It may be ideal," Jain says. "But 38 weeks is not term gestation in my mind, and women need to be very careful with that. When a vaginal birth occurs at 38 weeks it is very different from an elective C-section at 38 weeks. And the decision to undergo C-section, especially at a mother's or an obstetrician's choice, with no medical indications, needs to be carefully thought through, and parents need to ask their obstetrician about the evidence for and against it."
Kirkeby Hansen and colleagues report their findings in the Dec. 12 online edition of BMJ.