Interesting question came up on Quora, and I thought I’d share. Someone asked what determines whether we get innies or outies. Having never covered this in anatomy, I decided to investigate. Spent a few hours in the library researching ob/gyn, anatomy, and physiology texts, and also had a talk with one of my anatomy profs while hunting for the answer, so here it is…

First, some background:

  • Doctors generally all clamp the cord at about the same place, so it doesn’t play off tying technique, and there isn’t time to ask for preference during delivery anyways, as it’s recommended that the cord be clamped a mere 20-30 seconds after the baby is out of the uterus.
  • We generally start with bellybuttons that resemble outies, but as the subcutaneous fat layer develops under the skin around the umbilicus, they “pop in” to become your typical innie.
  • There’s discussion on whether post-natal umbilical hernias cause outies; I don’t know much about this particular condition, but I’m a bit skeptical–hernias will inflate/deflate with simple breathing, and while I suppose they could increase the chances of an outie after treatment, I doubt it’s significant.

The physiologic basis is the baby’s vasculature!

Every baby comes into this world with an umbilicus that can be represented like so, where “||” represents skin and “=” represents cord:
||
=========
||

When the cord is clamped upon the baby’s birth, the umbilical arteries and veins that run its length naturally constrict in response and shrivel, causing the cord to atrophy and fall off. In most people, it ends up looking like this:
||
==
||

The cord falls off at about the level of skin, and the 2nd bullet point above takes care of the rest to create an innie:
|fat|
==
|fat|

In some people, however, a longer chunk of their umbilical vasculature stays open, keeping slightly more tissue perfused and alive:
||
=====
||

You can see how this would create an outie:
|fat|
=====
|fat|

What factors encourage that extra skin for the outie to stay, no one knows, you might as well ask why most people are right handed but some people end up left. :-)

Bellybuttons: innie or outie?