You can introduce solids any time between 4 and 6 months if your baby is ready (see "How will I know when my baby's ready," below). Until then, breast milk or formula provides all the calories and nourishment your baby needs and can handle. His digestive system simply isn't ready for solids until he nears his half-birthday. Waiting until your baby is ready greatly reduces the risk of an allergic reaction and shortens the transition time between spoon- and self-feeding.
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A good rule of thumb is to start with rice cereal, which is gluten-free and less allergenic than other foods. First, nurse or bottle-feed your baby. Then give him one or two teaspoons of dry cereal mixed with enough formula or breast milk to make a semi-liquid. Use a rubber-tipped spoon when you feed your baby, to avoid injuring his gums. Start with just a small amount of cereal on the tip of the spoon.
If your baby doesn't seem very interested in eating off the spoon, let him smell and taste the cereal or wait until he warms up to the idea of eating something solid. Don't add solid food to your baby's bottle or he may not make the connection that food is to be eaten sitting up and from a spoon.
Begin with a once-a-day feeding, whenever it's convenient for you and your baby, but not at a time when your baby seems tired or cranky. Your baby may not eat much in the beginning, but give him time to get used to the experience. Some babies need practice keeping food in their mouths and swallowing.
Once he gets used to his new diet, he'll be ready for a few tablespoons of cereal a day. As the amount he eats increases, gradually thicken the consistency of the cereal and add another feeding.
Good luck and best wishes to you and the baby.