Third trimester ultrasound

What are some reasons drs will do another ultrasound after 30 weeks? I never had a glucose drink test I had blood work and was just over where he wanted me to be but hasnt had me do anything different. I really want another I'll be 32 weeks at my app next week and the women on my side of the family are notorious for having big babies so I wanna make sure she not gonna be a giant so I can maybe opt for an induction