Giving newborn baby boys antibiotics could stunt their growth in the early years of life, a new study suggests. Antibiotic treatment within 14 days of birth is linked with reduced weight and height in boys – but not girls – up to the age of six, experts found. The researchers think the differences may result from changes in the development of the gut microbiome – the trillion-strong community of beneficial microorganisms. However, antibiotics administered to babies older than 28 days could have the opposite effect, the research team also found. Antibiotic use after the neonatal period (the first 28 days of life) but during the first six years of life was linked with higher body mass index, in both boys and girls.