Autumn babies are more likely to develop asthma because they are exposed to lots of flu and cold viruses at a bad time, according to a study published in the American Journal of Respiratory and Critical Care Medicine.

Pingsheng Wu and colleagues at Vanderbilt reached this conclusion after examining Medicaid claims from 95,000 Tennessee children born between 1995-2000.

Infants born 4 months before the peak of the winter virus season had a 29% higher risk of developing asthma than those born 12 months before the peak.

Seasonal birth risk was at least as powerful an asthma predictor as established risk factors like maternal history of asthma, exposure to cigarette smoke, birth weight, gender, race, and the presence of siblings, all of which were controlled for in this study.

The scientists speculate that the seasonal birth link is mediated by a period of mild immune susceptibility that normally occurs 4 months after birth. This is the time when the infant’s stock of protective maternal antibodies has just about disappeared, and its ability to manufacture antibodies on its own remains somewhat limited.

It also may be that viral respiratory infections at this age (which are more common in winter) are more likely to impact airway reactivity and immune responsiveness in later life, according to the scientists.

The observation of a seasonal birth link to asthma, compelling though it may be, does little to settle the debate about whether early infections cause asthma or represent a sign of asthma predisposition, according to Renato Stein, who wrote an accompanying editorial.

“This is a controversial issue because children who attend daycare centers, and thus who are more exposed to recurrent respiratory infections, have been shown to have less asthma later in life,” Dr. Stein wrote.