I suspect that MIL might feel differently about the baby once it is born. So might your H. I mean, this child is likely to be just as wonderful as your two existing DC, all it means is that you will be a family of five rather than four.
You haven't said how old your oldest two children are. I appreciate that if they are school age, especially secondary school age and above then he may have felt the family was complete and is having trouble adjusting to the idea of nappies, night feeds etc again. However, once faced with a child who, just like his older two, is a little person in their own right and will be part of your family, he ought to get over the shock of the idea of a new baby when faced with the reality of looking after his new son or daughter.
If he doesn't come around to the baby once it is clear that it is staying, then I think this is unlikely to be just about having 3 children rather than 2. He maybe wasn't as committed to you and your existing children as he appeared to be.
I find it difficult to believe that a person would walk out on their partner and two children just because a third comes into existence. How is he going to explain that to the children when they are older? Talk about fueling sibling rivalry.
If he is worried that the pair of you don't have sufficent resources to take care of three children (in the manner the first two have become accustomed to) then how much worse is he making it for your elder children in terms of financial and emotional resources by seperating from you?
By the way though, you might find it easier to look after three children alone than you already find it to look after two children and a man-child.
Don't feel the need to stress that you are pro-choice. Choosing to keep the baby is entirely pro-choice. It's only anti-choice if you demand that every woman has to do the same as you.
I cannot believe that the father and grandmother of this child would pressurise his/her mother into aborting him/her when they can already see how great his/her siblings are. 
I'm sure your children will love their little brother or sister. Take care of yourself and make sure you speak to people who will support you :)