Heredity, nutrition, health, and environment all play a role in a baby's growth and development. Babies grow rapidly in the first year, usually tripling their birth weight. Babies must be handled carefully and must never be shaken. Feeding babies breast milk or formula meets the nutrition needs for the first six months. Other foods should be introduced gradually. A baby's clothing should be comfortable and easy to put on and take off. Babies should be bathed regularly but never be left alone in the bathtub. Parents should follow a recommended schedule of checkups and immunizations to protect the health of their babies.