Anatomy scan questions

Okay so I had my anatomy scan on Friday .. I went in thinking I was 18w2d (based off first trimester u/s). My doctor kept saying I was 19w1d based off of my LMP (I never went by this after my u/s). When I was in the scan the tech remarked at how big my baby was! She said "wow he measures several weeks ahead". When I looked at the screen it said 20w2d from measuring his leg. She told us the baby weighs 11oz.

I am curious how accurate this could be? Can the baby all of a sudden be 2 weeks larger?

I am still considering myself at 18w. The tech said everything was okay but because of positioning she wasn't able to get all she needed of his heart. I have to go back in a few weeks, but when I scheduled they told me the scan I would be going in for is a level 2 scan with a perinatalogist. My baby has had all normal/low risk test results but I do have hypothyroidism which they refused to treat until this past visit. Previously was seeing a midwife who ignored all my test results. I am a high risk because of my thyroid. Does anyone have any experience with level 2 scans? Should I be worried?

My doctors office is not the best at answering questions or even the telephone. I know none of you are doctors. Just curious of your experiences