Researchers in the Netherlands report that children born by cesarean section are nearly twice as likely to be diagnosed with asthma by age eight as children born vaginally. The link between cesarean sections and asthma was strongest in children who had two parents with allergies.

Researchers followed 2,917 children born between May 1996 and December 1997, of whom 247 (8.5%) were born by cesarean section. Asthma symptoms, an asthma diagnosis made by a physician, and any treatments were reported yearly by parents. By age eight, 12% of children had asthma, and the rate was highest (22%) in children with two allergic parents. Among children born by cesarean section, those with one allergic parent were twice as likely, and with two allergic parents three times as likely, to have asthma as children born vaginally. The results suggest "a strong interaction between genetic factors and the mode of delivery on the development of asthma," write the authors.

Just how cesarean section influences the risk of asthma is unclear, but early exposure to microbes may be involved. Children born by cesarean section aren't exposed to the mother's vaginal and intestinal microbes, first author and pediatrician Caroline Roduit told AJN. Early exposure to bacteria and viruses primes the immune system to produce antibodies that fight infections, whereas delayed exposure encourages the development of immune cells that set off allergic reactions. This "hygiene hypothesis" helps to explain the rise in asthma, eczema, and other allergic diseases in developed countries.

Cesarean sections are often performed for nonmedical reasons, and rates have risen dramatically in developed countries in the past few decades, as have the prevalences of asthma and allergies. A pregnant woman contemplating a cesarean section for convenience "should be informed of the risk of asthma for her child, especially when the parents have a history of allergy or asthma," the authors write.

Carol Potera

Roduit C, et al. Thorax 2009;64(2):107-13.