My girlies grew in the 30-40% of singletons up to 28w & at 28w they were about 2lbs a piece, at 31w babies A & C had not grown since 28w & were born at 2lb 2oz & 2lbs 8oz. The u/s measurments are only as good as the tech taking them (my u/s tech was off by less than 4oz on each kid 1 day before delivery).
A lot of Dr's are uncomfortable with doing vaginal birth with multiples & will automatically opt for a c-section because it's "safer" FOR THE DR. I was told at 8w I would have a c/s no way around it. But when I was 10cm dialated & in full labor (though no one new that except the babies lol) the Dr gave me a chance to consider going vaginal. We didn't because my girls were not in an ideal position (one vertex one breach & one transverse facing the wrong direction to follow her vertex sister out) & being as I had never considered it a possiblity I didn't press the issue. Looking back I am glad I did the c/s because of DD#1's cord knot but regardless I still feel sad that I never had the full birthing experience. I hope one day to convince DH to have another to remedy that though. My peri told me if I made it to 34w we would schedule me for delivery at 36w (average gestation for triplets is 32w).
Here's a tip for clothes I learned (I never got big but I know a lot of moms who have)- instead of maternity shirts wear maternity dresses, stretchy ones, with pants or leggings underneath. This way no matter the dresses intended length it's going to cover your whole belly & if it looks more like a shirt than a dress when it's on you've got the pants/leggings to keep your bottoms covered