I feel for you OP. It is so sad when moms are judgemental of their daughters' appearances at such a time, rather than being encouraging and celebrating a new life.
Of course, it may be right to intervene if there is a serious health concern, either for the daughter or the unborn baby - as in "Do you think your unborn baby enjoyed that cigarette?" or "Maybe three Mojitos before dinner is pushing it". But if it is just for appearance, then they should back off.
A couple of weeks after my DS was born my mom wanted to take the whole family, including her first grandchild, out for a smart Sunday brunch. I remember getting ready that morning, putting on make-up and taking care over my hair and generally feeling groomed and human for the first time since my DS was born. I even managed to squeeze into a pre-pregnancy outfit, and I must have been glowing when my DH told me I looked beautiful. Then mom comes out with "Are you going to wear a jacket over that?" "No" I said. "It's not cold." She replied "It's just that your buttons are straining a bit." I was devastated and all the good feelings I had built up for the event just crashed to the ground. Maybe I was being oversensitive (hormones!) and, sure, everything about my body was a bit bigger than before but I had looked in the mirror and had been pretty pleased at what I saw. Maybe she felt entitled to come out with that, as she was paying for brunch, but was that comment really necessary? I'll never know, because I didn't ask her and spent the day feeling completely deflated.