How accurate is the ultrasound due date?

They are accurate within 2 or 3 days. But this does not mean that you will have the baby on your due date. Babies can be born any time between 37 and 42 weeks. That is what midwives are taught anyway. With my first my ultrasound said I was due 10 days after my period date but my husband had been away on a trip form the day after conception! Second time around the ultrasound agreed completely, and showed twins. (now 16) == Your last known period date or the date of conception is a better predictor....as all babies are different & you may have an extrememly large or an extremely small baby for it's gestational age, and when compared against babies in the "norm" the due date could be way off.