Lacking certain gut microbes in the first three months after birth can put babies at risk for asthma, a new study shows.

Children who had low levels of four types of bacteria early in infancy were more likely to be diagnosed with asthma by age 3 than tots who had more of the microbes in their feces. Researchers in Canada report the findings in the Sept. 30 Science Translational Medicine.

By the time the babies were a year old, though, the researchers couldn’t detect any distinct differences between the gut microbes of 22 babies at high risk of asthma and 297 babies at low risk.

Those findings indicate that babies need certain bacteria — Lachnospira, Veillonella, Faecalibacterium and Rothia — in the first 100 days after birth to properly train the immune system and protect against asthma, the researchers conclude.

Mice inoculated with bacteria from a baby in the high risk group had offspring with severe lung inflammation. But when the researchers added the four mostly missing microbes to the shot of baby bacteria, the offspring’s lungs were less inflamed.

That result suggests that giving babies a cocktail of the microbes could lower asthma risk, although the researchers say they know too little about the microbes to attempt that now. Doctors may also be able to analyze newborns’ diapers to learn which babies are at risk of asthma.