Measuring behind

Kara • Married 04/24/15 -- 🇨🇦SAHM -x- Beautiful girl 09/09/16 -x- -x- Rainbow due Aug 2019 -x-

My uterus measures at 28 weeks and I am 31 weeks along, my baby is active with a 150bpm heart rate but I'm worried ... Having a growth ultrasound this week, how worried should I be?

Could this be because of genetic abnormalities??