1. Why do babies spit up milk? 1.1 Physiological reasons: Infants’ gastrointestinal organs are not mature enough and their digestive abilities are weak, making them prone to food reflux. In addition, if the feeding posture is incorrect, it can also lead to spitting up. 1.2 Overfeeding Some parents will overfeed their children in order to make their children grow faster. This can easily lead to the child eating too much, causing the stomach to become overcrowded.