UNCERTAIN DATES?

I had Hyperemesis Gravidarum during the first six months of my pregnancy. When I was eight months pregnant, I had a sonogram, it was a fully formed fetus, I saw the legs and feet moving and I saw the head and facial features and everything. I even heard the heartbeat. Now on the sonogram the baby is measuring 4 weeks smaller. They took a blood HCG qualitative test and told me I was exactly eight weeks according to dates and the tests. But now they are telling me according to the sonogram, the dates i gave them were four weeks off. I am wanting to know if my severe nausea and vomiting during the first six months affect the growth of my daughter? I am measuring at 32 weeks on sonogram, when I am 36 weeks according to dates. My baby is in head down position and has already dropped last week. Is it possible that I was wrong, or is the sonogram wrong? Because in the beginning of my pregnancy at eight weeks, i heard her heartbeat, saw the fully formed body on the screen
(She was a small fetus at 8 weeks, of course, and I know at 3 or 4 weeks, It would not be a fully formed fetus yet!) PLEASE HELP! I have not begun to dilate or have any blood show yet. I have a lot of fluid around the baby still... the baby is healthy they tell me but i am concerned about how she is growing. Which should I follow, Sonogram, or the blood tests done at eight weeks?