The gut microbiome during infancy interacts with a number of immune, endocrine and metabolic pathways that mediate early-life growth. Professor Ruairi Robertson explains that it is known that microbiome exposures during childhood, such as antibiotics, have varying effects on weight gain and growth in different settings. An immature development of the gut microbiome during the first years of life is associated with poor growth. Growth trajectories in the early years are major predictors of childhood undernutrition, obesity, cognitive development and longer-term chronic disease. This content was presented in May 2021 and reflects the best available information about Covid-19 at that time.