When a person eats, food passes from the throat to the stomach through the esophagus. The esophagus is called the food pipe or swallowing tube.
Once food is in the stomach, a ring of muscle fibers prevents food from moving backward into the esophagus. These muscle fibers are called the lower esophageal sphincter, or LES.
If this muscle doesn't close well, food can leak back into the esophagus. This is called gastroesophageal reflux (GERD).
In infants, a small amount of gastroesophageal reflux is normal. In fact, more than half a babies will have reflux during their first three months.
Persistent reflux with frequent vomiting leads to irritation of the esophagus and fussiness in the infant. Reflux associated with weight loss or reflux that causes breathing difficulty is considered abnormal.