It might not be fun, but checking the contents of your baby's nappy is can tell you so much about the health of your baby. Your baby's poo will look and smell different depending on if they are breastfed or formula-fed. Breastfed babies’ poo is runny and doesn’t smell offensive. Formula-fed babies' poo is firmer, darker brown and smellier. Some infant formulas can also make your baby's poo dark green. If you change from breast to formula feeding, you'll find that your baby's poos become darker and more paste-like.