My daughter sees a gastrointestinal doctor and I commented one time that it looks like she has an outie and he said that all belly buttons are innies, but some are just herniated and that is what causes an outie.
Indeed however appears the question is talking about in general. Not exactly during a pregnancy. Never-the-less it’s not important and we shouldn’t argue over something so minute.
I seem to remember being told when I was a kid that it has to do with how far from the belly the umbilical cord is tied off on an infant. I’ve never really investigated the veracity of that, though.