Twins measuring different at 34 weeks

At my ultrasound today they measured the babies and baby A measured 5.2lbs and B was only measuring 4lbs. They do ultrasounds at everyone of my appointments and the babies have always measured really close to the same so this worries me that all a sudden they're measuring this different. Does anyone know any reasons why one would be growing bigger than the other? I don't see my doctor till Wednesday and they said I'd have to wait to talk to her then.