You're generally born with it. A hernia is where there is a gap in the abdominal wall. It's usually picked up in babies and they're given surgery to close the gap. When pregnant the outer abdominal muscles part down the middle, widest at the belly button, which can expose a hernia. From what I've read it doesn't really seem to be a particular concern. I think there's a very small chance of a big of uterus getting trapped or something like that but I don't remember the circumstances behind that so wouldn't want to assert anything incorrect. I suggest you have a google!
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