They are really a guesstimate.....babies come only when they are fully cooked, due dates are misleading.
Also I think they say the earlier ultrasounds are more accurate.
First of all, why worry about just a couple days younger?? If they were off by weeks, I'd be worried. Second, I think they are accurate b/c they look at very specific structure growth that can be timelined (ex. major organs appear certain ways and function or not only past certain time frames) at 20 weeks. Be happy all the structures apparently looked good along w/ amniotic fluid, positioning , etc. And remember, your general due date probably hasn't changed...
it can vary by a week or more. the earlier ones are more accurate, but keep in mind every baby grows at its own rate. an ultrasound guesses age based on a generalized scale of what the baby "should" be at a certain age. not taking into account that babies vary in growth. i think it's fine. my twins have been anywhere from 5 days apart in size to the same day in size , back and forth.
The Earlier Ultrasounds are more accurate. I dont think you have a thing to worry about! be happy! You have a beautiful baby growing inside you! YAY!
During my 20 wks u/s with my first pregnancy, they told me he is gonna be a big boy, at least 91/2lb, He was just 6lb 15oz!!! So, you never know, when the fetus is ready , he will come out to meet us.