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Soaring cesarean section rates in many middle- and high-income countries have prompted questions about the potential effects of the procedure on children—such as the possibility of increased risk of allergic and certain autoimmune disorders.

Studies for years have hinted that babies born by cesarean section (C-section) might be at increased risk for asthma and type 1 diabetes. Enough data has accumulated for researchers to perform meta-analyses—large-scale analyses of existing studies.

In a study published this past May, Chris Cardwell at Queen's University Belfast in Northern Ireland and his colleagues combined the data from 20 studies and adjusted for factors such as maternal diabetes and breastfeeding. They concluded that cesarean section raises the risk of type 1 diabetes by 20% (Diabetologica 51, 726–735; 2008). They also crunched the data from 23 studies and showed the same increased risk for asthma—20%—in children delivered by C-section (Clin. Exp. Allergy 38, 629–633; 2008).

“It's a highly controversial area,” says Bruce Valance at the University of British Columbia in Vancouver. “Some studies find an association and others show no link. My guess is it probably does exist.”

The reason for these observations might originate with the gut. Babies born by cesarean section do not pick up microbes from the birth canal as they pass through. Research in mice has suggested that microbes literally consumed during a vaginal birth can help the immune system become tolerant to later exposures that might promote allergic or autoimmune disease (J. Exp. Med. 203, 973–984; 2006).

Such observations dovetail with preliminary findings presented by University of California, San Francisco researcher Ngoc Ly at the American Thoracic Society's 2008 international conference in Toronto. She and her colleagues examined immune cells in cord blood from babies born by C-section and in babies born vaginally and reported that babies born by C-section had reduced function in their regulatory T cells—cells that can modulate the immune response.

The rate of cesarean section rose by nearly 50% in the US from 1996 to 2005, when it reached 30.2 percent—much higher than optimal limit of 10–15% recommended by the World Health Organization.

Other countries with rates above 20 percent include Canada, the UK, Mexico and Brazil.