According the Mayo Clinic, a herniated belly button, more commonly known as an umbilical hernia, is the result of the intestines, fluid or other tissue protruding through weak or malformed abdominal muscles. This causes the belly button and surrounding area to bulge outward.
An umbilical hernia is common in infants due to their abdominal muscles not being fully formed, according to Healthline. It is most noticeable when the baby cries, since the act of crying, yelling or otherwise forcefully expelling air from the body causes additional pressure in the abdomen.
Healthline elaborates that umbilical hernias can also occur in adults and are more common in women who have been pregnant due to the strain pregnancy puts on the abdominal muscles, stretching and thinning them out. Being overweight or having a medical condition in which excessive amounts of fluid are present in the abdominal region can also cause strain that results in an umbilical hernia.
Umbilical hernias in babies are usually painless and recede as the child's muscles strengthen and the weak spots close up. According to WebMD, however, if it does not resolve itself, surgery is often required. This is true for adults with umbilical hernias as well because, if left untreated, the hernia can be incredibly painful and can strangle the trapped intestine, causing much worse complications, such as sections of dead intestine and blockages.