Newborns are defined as infants from birth up to 30 days old. At this age, they often experience a phenomenon known as “physiological milk overflow,” which tends to decrease as they grow. This phenomenon typically ceases around 7 to 8 months of age.
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Newborns often spit up (milk is easily expelled), which can cause concern for parents, but it is generally not a serious issue. Milk is ingested through the mouth, travels down the esophagus, and enters the stomach, where it is mixed with gastric juices for partial digestion before moving to the small intestine for complete digestion.
The lower esophageal sphincter is a valve located at the entrance of the stomach, adjacent to the esophagus, while the pyloric sphincter is located at the exit of the stomach, connecting to the small intestine. In young children, the horizontal position they typically maintain means that the stomach is situated higher, and it only becomes vertical when they begin to walk. Additionally, their nervous system is still immature, and the muscles of the stomach valves are weak and do not function in harmony, particularly the lower esophageal sphincter. The muscles of the pyloric sphincter are more developed and tend to close tightly due to stimulation after eating, which leads to frequent spitting up in infants.
Furthermore, due to the underdevelopment of the muscle layer, an infant’s stomach can easily become distorted after eating or swell up when swallowing air. Typically, the stomach capacity of a newborn is 30-35 ml, which increases to 100 ml at 3 months and 250 ml at one year. Therefore, each feeding session should be moderate (with a feeding time of about 15 minutes per session). In cases where mothers produce an abundance of milk, the duration of breastfeeding should be shorter, as overfeeding can also lead to spitting up.
