due dates

So i started of my pregnancy with one dr and when I was 7weeks6days and moved to another dr for personal reasons at 18weeks. Question is has anybody done their sono and their baby is of a by a week or two on the machine?? My dr says it's fine everything is normal and both baby and I are healthy. Also she wants to do another sono to see if it stays just one or two weeks and not increase. Anyone been through this??  Oh I'm 24weeks tomorrow.