The reasons for spitting up in 20-day-old babies are as follows: 1. 20-day-old babies have an underdeveloped digestive system, their stomachs are horizontal, their stomachs have a small capacity, their cardia sphincters are relaxed, and they will spit up when they take in a little more milk. 2. Improper feeding methods, babies often inhale air when they eat milk because they cry, which causes milk to flow backwards into their mouths easily and causes spitting up. If your baby vomits too much or in too large an amount, or even spits up too quickly, it is recommended that you do not feed your baby too much, eat less and more often, lift your baby’s head up when feeding, put it down again after a few minutes, or gently pat your baby’s back to burp it up. 3. Such as digestive tract malformation, encephalitis, etc., should be promptly checked at the hospital to clarify the cause.