26 weeks with Twin Boys

Stephanie
I am 26 weeks today. I had an ultrasound Tuesday and everything was great with the babies. They are both big in size measuring 2.5lbs and 2.4lbs. Both measure about a week and a half ahead and my belly measurement I was 31 weeks. I don't feel like I've gained enough weight (21lbs) and worried I won't make it to 37 week mark. Has anyone else experienced this?