When it is said that early (8 wk) ultrasounds are accurate +/- 2 days, what exactly does that mean? it is 1 to 2 days accurate of the conception day?

They are accurate: EDD are based on 40 weeks from last known normal menstrual period, or 38 weeks from known conception date.U/S dating is based on a database of average fetal anatomic measurements (size of head, abdomen, leg etc) for a particular age.The computer in the ultrasound calculates an EDD based on the average of these measurements. In the first trimester (

No: 1st trimester (7-13 weeks) US is generally considered accurate plus or minus 5 days for the gestational age of the baby.

Gestational age: Yeah. The ultrasound is used to estimate the "gestational age" of the fetus (How old the fetus is). Subtract that back and you will get the ESTIMATED date of conception

8 wk plus or minus 2: 8 weeks mean fetal age of 56 days, the ultrasound is not 100 % accurate, the result is +/ - 2 which means the real age of the fetus may be 54 (56-2) or 58 (56 plus 2) days