Ultrasound accuracy in dating pregnancy
The computer in the ultrasound (ignorantly) lumps your bigger baby into the dates of babies that big in the “normal” population.
This creates error in that the baby will compute out to be further along than he or she really is.
So in this arithmetic test, the baby sets the curve.
For many women, especially after 8 weeks gestation, sufficient information about the baby may be obtained with transabdominal ultrasound only.
However, in the early pregnancy, the developing embryo is very small (at 6 weeks gestation, the baby is only 5-9mm long) and a transvaginal ultrasound may be required to get a better image of the baby.