Once you get an estimated due date that comes from the size of the embryo in the ultrasound, you can either count back from it 266 days on a calendar
, or you can put it into an online conception
calculator, and it will give you an estimated conception date. But you need that due date to be based on the information from the ultrasound.
If a doctor tells you a certain number of weeks pregnant, ask if he is talking about how long since conception.