Every belly button falls into one of two broad categories: innies, which dip inward, and outies, which protrude. Roughly 90 percent of people have innies, making outies a clear minority. But within those two camps, navel shapes vary widely in depth, orientation, and contour, and the forces that shape them are less intuitive than most people assume. How your belly button looks comes down to the healing process after your umbilical cord separates, not how the cord was cut or clamped.
How a Belly Button Forms
The belly button is a scar. During fetal development, the umbilical cord connects a growing fetus to the placenta, carrying blood through three vessels (one vein and two arteries) embedded in a gelatin-like tissue called Wharton’s jelly. The cord’s basic architecture begins taking shape as early as the fourth week of fetal life, when the embryonic plate folds and a primitive umbilical ring appears on the front of the body.1SpringerLink / Front Med. Anatomy and embryology of umbilicus in newborns: a review and clinical correlations After birth, once the cord is cut and the stump dries out, the remnant falls off within a week or two. What remains is a patch of scar tissue where the cord once entered the abdomen.
The shape that scar takes is mostly determined by how the underlying tissue heals, how much subcutaneous fat surrounds the area, and how the skin contracts as it scars over. It has nothing to do with the doctor or midwife who clamped the cord. There is no special tying technique that produces an innie versus an outie. The popular myth that the way the cord is tied or clamped dictates belly button shape persists in casual conversation but has no anatomical basis. Cord clamps sit well above the skin, and the stump shrivels and detaches on its own.
Innies, Outies, and Everything in Between
The simplest explanation for an outie is a small umbilical hernia, where a bit of abdominal tissue or fat pushes through the opening in the muscle wall where the cord used to pass. These hernias are extremely common in newborns and usually close on their own during the first few years of life. When they don’t fully close, the belly button may permanently protrude. A less common cause is excess scar tissue or skin that heals outward rather than inward after the cord stump falls off.
Among innies, there is more variety than people realize. Some are deep and narrow, others are wide and shallow. Some sit vertically, others stretch horizontally. The shape depends on how much fat is in the surrounding abdominal wall, the width of the scar, the elasticity of the skin, and how the muscular fascia healed. People who gain or lose significant weight often notice their belly button change shape because the fat layer beneath it shifts. Pregnancy is another common trigger: the abdominal wall stretches dramatically, and an innie can temporarily or permanently become an outie as intra-abdominal pressure pushes the tissue forward.
What Shapes Are Considered Attractive
Cosmetic research has actually tried to quantify what people find appealing in a belly button, because the navel is a significant concern for patients undergoing tummy tucks and other abdominal procedures. A study analyzing aesthetic preferences found that a small, vertically oriented oval with a slight hood of skin at the top was the most appreciated shape, preferred by about 82 percent of respondents. By contrast, a horizontal oval shape was preferred by only 29 percent, and a protruding (outie) shape by 47 percent.2PubMed. Concepts in Navel Aesthetic: A Comprehensive Surface Anatomy Analysis
Surgeons who perform abdominoplasty pay close attention to these preferences. The features they aim for when reconstructing or repositioning a belly button during a tummy tuck include small overall size, a vertical or T-shaped opening, and what they call “superior hooding,” meaning a slight overhang of skin at the top that creates shadow and depth. A widened, flattened, or outward-turning navel is generally perceived as less attractive and can be a major source of dissatisfaction for patients after abdominal surgery.3PubMed Central. Umbilicoplasty in Abdominoplasty: Modifications for Improved Aesthetic Results This is why a fair amount of surgical technique literature focuses specifically on how to create or preserve a natural-looking navel.
Cosmetic Surgery on the Belly Button
Umbilicoplasty is the formal name for cosmetic belly button surgery. It can be a standalone procedure or, more commonly, part of a larger abdominoplasty (tummy tuck). During a standard tummy tuck, the surgeon separates the belly button from the surrounding skin, tightens the underlying abdominal muscles, pulls the skin downward, and then creates a new opening for the navel at the appropriate position. Getting the navel to look natural after this process is one of the trickiest parts of the operation, because a poorly placed or overly round belly button can give away that someone has had surgery.
Various techniques have been developed for this. Some involve creating small skin flaps around the navel site and folding them inward to produce depth and the desired superior hooding. One method known as the “four flaps technique” elevates four small sections of neighboring tissue and inverts them to build a new belly button with sufficient depth and a natural contour.4PubMed Central. Four flaps technique for neoumbilicoplasty Another approach involves defatting a cylinder of tissue down to the skin and suturing it to the rectus muscle so that it heals into a concave shape, essentially forming a new hollow.5Journal of Plastic, Reconstructive & Aesthetic Surgery. Abdominoplasty with customized transverse musculoaponeurotic plications A review of cosmetic umbilical procedures described at least eight distinct surgical approaches, noting that the more complex reconstructions are reserved for cases where the original belly button is too damaged or malformed to salvage.6The American Journal of Cosmetic Surgery. The Cosmetic Umbilicus: A Review of Aesthetic Features and Surgical Methods
Newer techniques continue to refine the results. A 2024 study introduced a novel umbilical reconstruction method that emphasized high patient satisfaction, with patients reporting the reconstructed belly button looked natural and aesthetically pleasing.7PubMed. Refining Abdominoplasty: A Novel Technique for Umbilical Reconstruction The minor complications that can follow these procedures include delayed wound healing, decreased depth of the navel over time, and occasional blood pooling under the skin.8PubMed. Umbilical reconstruction after repair of omphalocele and gastroschisis
The Belly Button Microbiome
Your belly button is home to a surprisingly rich community of bacteria. A large citizen science project that analyzed the bacteria living in people’s navels found that, on average, each belly button harbored about 67 distinct bacterial types, making it at least as diverse as other well-studied skin sites like the forearm or the back of the knee.9PubMed Central. A jungle in there: bacteria in belly buttons are highly diverse, but predictable The researchers also found a striking pattern: while the total diversity was enormous (hundreds of different species across all the samples), a small handful of bacterial types dominated. Just six species showed up in more than 80 percent of the people sampled. Those six accounted for a tiny fraction of the total species diversity but made up roughly a third of all the individual organisms detected.
Belly button shape likely matters here. A deep innie provides a warm, moist, sheltered environment where bacteria thrive. An outie, by contrast, is more exposed to air and friction from clothing, which may support a different microbial profile. The researchers noted that despite huge variation from person to person, the dominant species were predictable, suggesting that the navel’s basic physical environment selects for certain bacteria regardless of a person’s hygiene habits or geographic location. If you have ever noticed a mild odor from your belly button after ignoring it in the shower for a while, that is the microbial community making itself known.
Why Some People Get Belly Button Lint
Belly button lint, also called navel fluff, is a peculiarly common phenomenon that has attracted genuine scientific investigation. The accumulation is not random: researchers have modeled how it works and identified a surprisingly elegant mechanism. As you breathe, your abdomen expands and contracts, creating a slight sliding motion between your clothing and skin. Body hair on the abdomen acts like a one-way ratchet: the microscopic scales on each hair shaft point away from the root, which means fibers from your clothing can travel along the hair in one direction but not back.10PubMed Central. Modeling the production of belly button lint Since abdominal hair generally grows in a pattern that points toward the navel, loose clothing fibers are gradually funneled into the belly button over the course of a day.
This explains why belly button lint is overwhelmingly a phenomenon of people with more abdominal hair. One researcher confirmed the hypothesis by showing that when abdominal hair was shaved, lint collection stopped entirely. Chemical analysis of the lint itself revealed that it is mostly cotton fibers mixed with a smaller amount of skin cells, oils, and proteins.11PubMed. The nature of navel fluff The source fibers can come from the shirt worn closest to the skin or even from a towel used after a shower.12Scientific Reports. Modeling the production of belly button lint New shirts tend to shed more, which is why people sometimes notice more lint when wearing a fresh garment. The color of the lint usually matches the dominant color of the clothing worn that day.
Medical Conditions That Affect the Navel
Beyond cosmetic concerns, the belly button can be a site of real medical significance. One of the most common issues in newborns is an umbilical granuloma, a small, fleshy, pinkish mass that forms after the cord stump separates. It is the most frequent cause of an umbilical mass in the first weeks of life and develops when healing tissue over-proliferates instead of scarring normally.13PubMed Central. Salt Treatment for Umbilical Granuloma – An Effective, Cheap, and Available Alternative Treatment Option: Case Report Granulomas are benign and are usually treated with silver nitrate application, though simple salt treatment has also been reported as effective. They resolve without long-term changes to belly button shape.
A rarer but more serious condition is primary umbilical endometriosis, sometimes called a Villar nodule. In this condition, endometrial tissue (the type that normally lines the uterus) implants in or near the belly button. It causes cyclical symptoms tied to the menstrual cycle: the navel may swell, become painful, change color, or even bleed during menstruation.14PubMed. Primary umbilical endometriosis The condition is uncommon enough that it can be misdiagnosed as a hernia or a cyst, but imaging and tissue biopsy can confirm it.
Perhaps the most ominous belly button finding is what is known as a Sister Mary Joseph nodule, a hard lump at the navel that signals advanced cancer within the abdomen or pelvis. It occurs in roughly 1 to 3 percent of people with intra-abdominal or pelvic cancers. The most common primary sites are the gastrointestinal tract (about 52 percent of cases) and the genitourinary tract (about 28 percent), though in 15 to 30 percent of cases the original tumor is never identified.15PubMed Central. Sister Mary Joseph’s nodule: an unusual but important physical finding characteristic of widespread internal malignancy The nodule is named after a surgical assistant at the Mayo Clinic who first drew attention to the association in the early 1900s. Finding one generally indicates that the disease has already spread widely, so the prognosis is poor. Any new, hard, or growing lump at the belly button in an adult warrants prompt medical evaluation.
Congenital Anomalies and the Absent Navel
Some people are born with unusual belly buttons because of developmental conditions that affect the abdominal wall. Gastroschisis and omphalocele are two conditions where the intestines (and sometimes other organs) develop partly outside the body through an opening near the umbilicus. Both require surgical repair shortly after birth. Modern surgical techniques for gastroschisis can preserve a near-normal-appearing belly button, avoiding the need for a separate reconstructive procedure later.16Journal of Pediatric Surgery. Repair of gastroschisis with preservation of the umbilicus When the original navel cannot be preserved, surgeons may perform a reconstruction to create an artificial one, since the absence of a belly button can be psychologically distressing, especially for children and adolescents.17PubMed. Umbilical reconstruction after repair of omphalocele and gastroschisis
Other congenital anomalies involve structures that should have disappeared during fetal development but didn’t. The omphalomesenteric duct and the urachus are two fetal channels that connect, respectively, the gut and the bladder to the umbilicus. Both normally close and are reabsorbed around the tenth week of gestation. When this process is incomplete, remnants can persist and cause problems ranging from a persistently wet or draining belly button to cysts or fistulas that require surgical correction.18PubMed Central. Coexisting Remnants of the Omphalomesenteric Duct and Urachus in an Infant These conditions are uncommon but worth knowing about, particularly for parents of infants whose belly buttons continue to leak clear fluid or bleed weeks after the cord stump has fallen off.
Belly Buttons Across the Animal Kingdom
Every placental mammal that develops inside its mother with an umbilical cord gets a belly button of some kind. In large animals like dolphins, whales, and great apes, the scar is visible and roughly analogous to a human navel. In smaller mammals like dogs, cats, and rodents, the scar is present but usually tiny, flat, and hidden under fur, which is why most pet owners never notice it. Animals that develop inside eggs, like birds and reptiles, also have a navel of sorts, but it forms where the yolk sac was attached rather than where an umbilical cord entered, so it tends to be much smaller and disappears more completely as the animal grows. Marsupials like kangaroos, which spend most of their development in an external pouch, do have a belly button from their brief time attached to a rudimentary placenta, but it is essentially invisible in adults.
The one group that definitively lacks belly buttons is egg-laying mammals, the monotremes, which include the platypus and echidnas. Since they hatch from leathery eggs and never develop an umbilical cord, there is no scar to leave behind. So if you have ever looked at your belly button and wondered whether it serves any purpose, the answer is straightforward: it doesn’t. It is a healed wound that happened to form in a conspicuous spot, and the enormous range of shapes it takes is simply the result of how that wound happened to scar.