According to the AAP and the WHO, breast milk or formula should ideally be the only source of nutrition for a baby for the first six months of life. Babies who are given complementary foods earlier than that tend to wean earlier and mothers often report lower breast milk supply. The recommendation used to be four months, so many pediatricians haven't kept up with the current recommendations and given that suggestion, which can be confusing. Solids should not be given to increase weight gain or to help baby sleep through the night. Here is a great resource with all the current recommendations and research regarding the introduction of solids to breastfed babies - Starting Solids from Kelly Mom. Additional Reading: Tips for Introducing Solids