Last month, I covered a study about the prevalence of cesarean sections in the U.S. (And before that, Karen Gromada covered two studies about risks of so-called “near term” births and c-sections.) This month, I look at a study about a possible risk of cesarean delivery: higher incidence of childhood asthma for the infants.

A research team in Norway led by Dr. Mette C. Tollånes sought to explore a possible link between increase in childhood asthma and increase in cesarean delivery. To do so, they examined the records for a whopping 1,756,700 births entered into that country’s Medical Birth Registry (MBRN) over a twenty-one year period (1967 to 1998). They followed the infants up to age 18 or the year 2002.

Methods
From 1967 to 1987, MBRN classified births as spontaneous vaginal, instrumental vaginal (forceps, vacuum extraction), or cesarean section. Beginning in 1988, cesarean sections could be classified as planned or emergency based on available data.Data about childhood asthma was derived from records of social security cash benefits received for asthma, as coded by the child’s physician and entered into the National Insurance Scheme (NIS).

Results
Here’s what the researchers found:

• The rate of c-section deliveries increased during this period. C-section rates increased from 1.7% (1967) to 11-12% (1998). (Remember, these data are for Norway, not the U.S.)

• The rate of childhood asthma increased during this time, too. The cumulative incidence of asthma was 4.0 per 1,000. During the study period, children born by c-section had a 52% higher risk of asthma than those born by spontaneous vaginal delivery. Children born by instrumental vaginal delivery also had a higher risk (19%) than the children born by spontaneous vaginal birth.

• Emergency c-section deliveries had higher risk than planned c-section deliveries. Looking only at the data for those years during which data for emergency and planned c-sections could be separated (1988-1998), babies delivered by emergency c-section were at 59% increased risk, compared with 42% for those delivered by planned c-section.

• Risk of asthma for children born by c-section was higher for lower gestational age. The risk of asthma was higher for children born at less than 37 weeks of gestation than those born at 37-41 weeks or those born at 42+ weeks.

Study limitations
Unfortunately, the researchers did not account for some important factors, such as parental smoking and maternal asthma. Since their asthma data were drawn from payment records, it is possible that they may have been underreported.

Still, the study has several strengths, including its large, population-based scope and the lengthy follow-up period. Also, earlier studies have found the data sources to be reliable.

Take-home message
Dr. Tollånes and colleagues conclude that c-section delivery is “associated with a moderately increased risk of asthma,” and that the association is “apparently stronger for emergency c-sections than for planned c-sections.” However, the doctors are unable to determine why this is the case. They begin with two hypotheses (increased risk is due to a lack of exposure to maternal microbial flora the baby would experience during vaginal delivery; increased risk is due to respiratory distress during delivery), and neither is completely supported by their data.

Expectant parents are urged to talk with their providers—before the day of delivery—about the risks posed by a cesarean section. Those who have delivered by c-section are urged to be aware of the increased risk and watch their children for signs of asthma.