Early life corresponds to the first bacterial colonization of the digestive track, creating a microbiome that appears to play an important role in human development. what might be the effect of early exposure to antibiotics on children's development? one study followed 11,532 full-term babies all born in the county of avon (united kingdom) during 1991 and 1992. exposures to antibiotics during infancy were recorded, along with recurring measures of body mass over several years. the study found that antibiotic exposure during the first six months of life was associated with increased body mass later on, a pattern not seen with antibiotic exposure occurring after the first six months. the explanatory variable in this study is: