One of the body systems that takes the longest to adapt to life outside of mom's uterus is the digestive system. This is even truer for babies who are fed formula. Many people don't realize that it may take six to eight weeks for their baby's digestive system to settle down and "act right."
Many Reasons to Spit Up
Your baby's stomach is about the size of her fist. So, she doesn't need much milk the first few days. If her stomach is over-full, she will probably spit up. If she is working to get mucous out of her stomach (see Gagging), she will probably spit up. If she swallowed air while crying or sucking, she will probably spit up. Just because she is brand new, and the muscles of her stomach, esophagus, throat and mouth aren't coordinated yet, she will probably spit up.
Is It the Milk?
Many people think their new baby is spitting up because there is something wrong with the milk (see Milk Allergies). Usually, this is not the case. New babies often spit up because their digestive system just needs time to get used to working. The digestive system didn't have to do anything inside mom's uterus, and now it has to handle all sorts of new functions.
Thanks to Janelle Aby MD, Stanford School Of Medicine, Newborn Nursery, and Lucille Packard Children's Hospital for the use of occasional photographs.