I think it can be off about three weeks.

That far along, they could possibly (probably) be judging by size, like weight and such. This could just mean you have a small baby. I know a little girl who was born small, and now she's 2 1/2 years old and still wearing 12 - 18 mos. Clothes. She's almost the right size, growth wise, but she's skinny. So her dresses fit ok, except they're short (because they're a smaller size). But she's fine, healthy, and (not that i'm biased or anything ;d) smarter than normal children her age. (then again, she's got 4 older siblings to teach her things).