My last period was in October 31

💙💙💗Desiree💗💙💙 • Mommy of 2 boys and 1 girl 👦👦👑👣🎀They are my everything

Im should be 18 and 3 days pregnant. But the ultrasound tech at my gender ultrasound said my baby is smaller then 18 weeks and changed my due date to August 15 it was originally for Aug. 7th. Anybody on the same boat. I dont know if I should go by what she said or go by 17 weeks 3 or 18 weeks 3 days ?