How accurate can be a pregnancy scan in terms of delivery dates

 
Ante natal ultrasound scans can estimate the period of gestation with reliable accuracy. This is done by comparing various parameters measured on the foetus with standard values that are available. From the period of gestation, an Estimated Date of Delivery (EDD) is calculated. This date is only an estimate. The baby will be born when it wants to be. This natural process cannot be predicted. There are far too many variable that affect labour and delivery. In the majority of cases, however, the EDD is correct to within 7-10 days of the actual date of delivery.