Baby measuring small!

Soo I’ve been having routine ultrasounds every couple of weeks to monitor baby girl, soo today I had one and she’s still measuring small they said she’s about 3lbs, I have an appointment with a fetal specialist on the 13th of September to measure her! My OB did tell me there’s a possibility I might be delivering early! Any moms go through the same thing?! As far as I know she’s doing okay just really small! Just trying to be prepared and super nervous! What should I expect?