"girl123 - I suspect your MIL is always going to be opinionated, controlling and domineering."
That's a massive leap to make and not a very helpful comment. We don't know the MIL is like this, all we know is that she is upset at this situation and IMO that is not unreasonable.
Her son is young, so is the OP, they have not been together very long, they have no home and are probably of limited means and they are going to become parents.
Now this is not always a barrier to a successful partnership or family life, but often it is. I think the mother is allowed to be worried.
She may have gone over the top in calling for an abortion but an abortion isn't going to happen is it. The OP is going ahead with the pregnancy and the mother is going to be a grandmother whether she likes it or not.
However, once the baby arrives she may be over the moon and want to help but my worry is that the OP is going to stick to her guns, continue to resent the hell out of the mother for expressing an opinion and brand her as opinionated, controlling and domineering for the rest of their relationship.
This is no way to proceed if only of the sake of the baby.
My advice OP is that you do the grown up thing, bite the bullet and go and see the mother. Tell her how upset you were by the abortion comment but request that for the sake of the baby you all work together and get along.
Open an honest dialogue with her. Go direct, don't use her son as a go-between. If you have this baby you and her are connected.
It's a big, brave step to take and you have to put your ego aside to make it. The pay off could be great though, a supportive relationship with your "MIL". If it isn't and then by all means post again and say "told you so".