For those of us who have “innie” belly-buttons, what is keeping our belly-buttons “pulled in” that causes them to stay indented on our stomachs without eventually flattening out?

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For those of us who have “innie” belly-buttons, what is keeping our belly-buttons “pulled in” that causes them to stay indented on our stomachs without eventually flattening out?

In: Biology
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It’s entirely random, however only about 10% of the population ends up with an ‘outie’. It has to do with how much skin makes it onto the umbilical. If there’s a lot, then it stays out. If it’s ‘normal’ then it eventually grows in. Doctors can’t control it.

The umbilical cord extends through the skin, and shrivels up on the inside because it is no longer needed after you are born. This puts a little tension on the button, “pulling” it in. As your belly grows the shriveled part does not grow too, which increases the tension. Often pregnant people do get flat belly buttons when the skin is stretched.

As you might know the belly button is what is left of the main blood vessels (and other important tubes) that connected a baby to the placenta of the mother. As that tube stops being useful it basically shrivels and turns into a tendon. This tendon connects the outer skin layer to the inner layer of the abdominal cavity. That is what is “pulling” the innie belly button down. I can also try explaining why outies happen (spoiler alert umbilical hernias) but i am trying to keep this as eli5 as possible. Let me know if you have any other questions.

Also fun fact. Belly buttons can fail to close and leave a tube connecting the bladder to the belly button and pee comes out (patent urachus).