The belly button, or umbilicus, is a universal feature and a permanent reminder of a person’s earliest connection to life. Despite the common misconception that its shape is determined by how the umbilical cord is cut, the umbilicus is simply a scar left from a natural biological process. The majority of people possess an “innie,” or inverted navel, leading to curiosity about the specific biological mechanisms that create this distinct indentation. The shape of the belly button is primarily an outcome of individual healing and the structure of the underlying abdominal tissue.
Understanding the Umbilicus
The umbilicus marks the point where the umbilical cord once passed through the abdominal wall. During gestation, this cord functions as the fetus’s lifeline, containing blood vessels that facilitate the exchange of oxygen and nutrients from the placenta to the developing baby. It also carries waste products away from the fetal bloodstream back to the mother’s circulation.
Immediately after birth, the cord is clamped and cut, leaving a small umbilical stump attached to the newborn’s abdomen. Over the next one to three weeks, this stump undergoes a process of tissue atrophy, drying out and shriveling as the vessels inside naturally close off. Once the dried-out stump falls away, the resulting healed area is the umbilicus, a scar that develops based on how the remaining internal and external tissue retracts.
Why the Healing Process Leads to an Innie
The “innie” is the most common result, accounting for approximately 90% of all belly button shapes, because the natural healing process involves internal retraction. The blood vessels within the umbilical cord—two arteries and one vein—turn into fibrous, internal ligaments after the cord is cut. This internal scarring naturally pulls the skin and external scar tissue inward toward the abdominal wall.
This inward pull, or scar retraction, is amplified by the way the skin adheres to the deep layer of connective tissue, known as the fascia, at the base of the navel. A strong, tight adherence to the abdominal musculature ensures the skin is pulled into a depression, creating the inverted appearance. The amount of subcutaneous fat present around the abdominal wall also influences the depth of the inversion, making the surrounding tissue more prominent than the recessed navel. An “outie” typically forms when the skin adheres less tightly or if excess scar tissue forms on the surface, pushing the navel outward.
When a Variation Is Not Normal
While most variations are merely cosmetic, certain deviations can cause the belly button to protrude. One frequent cause is an umbilical hernia, which occurs when the small opening in the abdominal muscles fails to close completely. This allows a portion of the intestine or fat to push outward when the baby strains or cries, creating a pronounced, soft bulge.
Umbilical hernias are common in infants, affecting up to 20% of newborns, but they are usually painless and often close spontaneously by the age of three. Another deviation is an umbilical granuloma, a small, moist, reddish-pink lump of excess granulation tissue that can form after the cord stump falls off. This tissue is not sensitive but may weep a yellowish discharge. It sometimes requires a simple application of silver nitrate for treatment. For the majority of people, however, the innie or outie shape is simply a benign, unique anatomical outcome of the body’s healing process.