Measuring behind

Kara • Tue, Jul 05 • Married 🇨🇦 SAHM Girl 09/09/16 -x- Boy 08/29/19 Waiting to TTC #3

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??