I think it tends to be jealousy, or, as SGB says, that before the baby comes along your expectations and needs in the relationship are different. You're probably fairly equal in standing, neither of you relies much on the other, when he is demanding of your time, resources or affection you give them willingly because you have plenty to give and also, you love him, so of course you're forthcoming with these things.
When the baby arrives, the relationship is no longer the centre of your world or sole focus of your attention (which is a fairly unhealthy thing for a relationship to be anyway, but society tells us this is normal - see Twilight and other examples of romance in popular culture) - by the way, the abuse/jealousy tends to step up if your focus is diverted from the relationship/abuser in other ways too such as work, an intensive hobby or certain friendships - it's just that if you are blinded enough by the relationship, often we consider these an acceptable sacrifice :( rather than realising that it's the relationship which is at fault.
So, not only are you very suddenly in a situation where your focus, time and affection are taken away from the relationship in a way you simply can't compromise or sacrifice, you're also in a very vulnerable situation where you're relying on your partner for emotional and practical support. Contrary to the posts above, abusers hate this. They are generally crap at emotional support (because this requires empathy, and if abusers had empathy, they wouldn't abuse) and also they don't like giving away their precious resources to you, not constantly. Maybe occasionally as a display of how much they must really care, deep down (ie, this is what you take from it - they don't really care that much deep down or they'd show it when you needed it most.)
This shows itself too when you are ill - abusers don't cope very well when their partner is ill, they get frustrated, are impatient and basically act as though you've gotten ill on purpose just to inconvenience him.