It is also not out of the question of how much a 9-month-old baby weighs and a 9-month-old baby weighing 7.5 kg meet the standards or not? According to the World Health Organization (WHO), 9-month-old boys have an average weight between 8.1 and 10 kg, and 9-month-old girls have an average weight between 7.3 and 9.3 kg.