Is the due date on an ultrasound calculated on 40 weeks or 38 weeks?

Your due date is calculated by the first day of your last regulare menstrual period the doctor will give you an estimated due date according to the chart that they have which is calculated to 40 weeks....the ultrasound will give you an estimated due date according to your LMP and fetal development if the fetus looks under or overdeveloped for when your saying your LMP was then your doctor will consult you and maybe run some more tests but all EDD are to 40 weeks although remember they give you a 2 week margin meaning you could go at 38 or 42 weeks hope this helps Good Luck and God Bless!!!