anatomy scan

My doc gave me a due date for November 1 but I've never had a 28 day cycle so I don't think he's right but he said he's gonna keep that as my due date. My baby has been measuring a week behind and Wednesday I go for my anatomy scan and if he's right I will be 19 weeks. If I go and the baby is measuring behind 19 weeks does that mean I'm not 19 weeks? He said about this time the baby should be catching up to his measurements I'm just so anxious to know bc I don't want my due date to be even further away so ready to see my baby💙