When you look at your new baby, it's easy to see that he's not quite finished yet. While he has all his essential parts and systems, they don't all function optimally in the first few months after birth. A baby's digestive systems work well for digesting breast milk, a food designed especially for them, or formula, which attempts to closely mimic breast milk. But babies can't handle solid food or large feedings in the first few months of life, because their intestinal tract isn't ready for them yet.
Your baby's digestive system, like the rest of him, isn't very big. Because he has a tiny stomach about the size of a marble at birth, he needs frequent feedings. After several days, his stomach expands to the size of a ping-pong ball but still holds just 1.5 to 2 ounces at a time, according to the Similac website. As he grows, so does the amount of food he can handle at one time, so he can go longer between feedings. Because he needs to take in a large amount of calories in a relatively small volume of food, breast milk is best because it has a high fat content as is formula. Fat supplies twice the number of calories as protein or carbohydrate, gram for gram.
A baby's intestines lack the protective mucosal barrier that helps seal off the intestines, decreasing the risk of both bacteria and potential allergens permeating through the intestine into the bloodstream. For this reason, a baby has less risk of developing an allergy to foods if he sticks to breast milk or formula until age 4 to 6 months.
The pancreas of newborns doesn't produce the enzymes such as amylase needed to digest complex carbohydrates or starches until around age 3 months, registered nurse Dr. Adele Pillitter explains in her nursing textbook, "Maternal and Child Health Nursing: Care of the Childbearing and Childrearing Family." Your infant also produces less lipase, which helps digest saturated fat, than he will after his first year of life.
You don't have to spend much time with babies under age 3 months to realize that more than 50 percent of babies spit up, according to the National Digestive Diseases Information Clearinghouse (NDDIC). The lower esophageal sphincter separates the stomach from the esophagus, the tube that leads to the mouth. This valve normally closes tightly to keep stomach acid and food in the stomach, where it belongs. But in infants, the sphincter isn't very strong yet, so it opens more easily than it will later in life. This allows a small amount of food to reflux, or pass from the stomach back into the esophagus, down into the mouth and right onto your baby's bib -- or your shirt. Babies with reflux usually stop spitting up between 12 and 24 months, notes the NDDIC. Babies who fail to gain weight or who vomit frequently could have gastroesophageal reflux disease, a more serious form of reflux; talk to his pediatrician.