As a father of three, I have a suspicion that a lot of people posting here are chasing a totally unreal state of affairs anyway. Because in the sense I think most people mean it, I don't think babies are actually incontinent.
Anyone who's actually spent time with babies knows very well the funny little face they make when they're pooping. They very often have to make an effort. SOmetimes they get quite red in the face. And peeing is the same - it doesn;t dribble out continuously; it builds up to a point, and then they let it go. Babies have sphincter muscles just like adults. They aren't incontinent - they just don't know any better. Toilet training consists of teaching them that when they need to go, they should go in the toilet. Not in their pants. Incontinence is a medical condition in which people lose control of their sphincter muscles. Babies simply haven't learnt yet that they have such control - but it's there pretty much from the moment they're born.
So what people are talking about is not really loss of control. It's more like loss of knowledge - or re-training. This is what Darkfinn means I think. You have to train yourself to believe that the diaper is the right place to go. BEcoming incontinent is actually a different thing altogether - and actually it isn't really what babies are anyway.