Researchers pooled data from 22 previous studies published between 1950 and July 1, 2010. Two of the 22 studies looked at antibiotic exposure during pregnancy while 19 studies evaluated antibiotic exposure during the first year of life. One study assessed antibiotic exposure during both time periods.
Other studies have shown that infants who receive antibiotics are at an increased risk for developing asthma by age 7, and the more courses of the drug given that first year, the greater the risk.
Nine million children under age 18 in the U.S. have asthma, according to the American Academy of Allergy, Asthma & Immunology.
What was unique about this current review is that it analyzed a larger number of studies and a larger number of participants (nearly 600,000). It also grouped studies according to design type (database, retrospective, or prospective) to see how that affected results.
When all 20 studies were grouped together, researchers found that infants who took antibiotics during their first year of life were about 50% more likely than babies who never received the drugs to be diagnosed with asthma.
But when researchers tried to take into account variables that could skew the findings, such as when antibiotics are used to treat respiratory infections that may truly be early symptoms of undetected asthma, the effect was much smaller. In studies that adjusted for these respiratory infections, a child who took antibiotics was 13% more likely to be diagnosed with asthma than a child who never took the medication.
Scientists have suggested an increase in antibiotic use has been linked with more cases of asthma and that the "hygiene hypothesis" may play a role. The hygiene hypothesis suggests that children who grow up in an overly clean environment with less exposure to bacteria may actually be more at risk for developing asthma because their immune systems may not develop optimally.
In a separate analysis, the children of women who took antibiotics during pregnancy were nearly 25% more likely to have asthma compared to mothers who did not take the drug.
The researchers are not suggesting that early antibiotic exposure causes childhood asthma, but even a slight increase in risk may be good reason to avoid unnecessary antibiotics during pregnancy and the first year of life.