Yes, the ultra sound measures the head and a forearm. Depending on the size is how they come up wth your conception date and due date. Your due date is typically 2 weeks before or even 2 weeks...
From your Last period date. The accuracy of the due date depends on how correctly you tell doctor about your last menstrual cycle ( monthly period) date. If you want more info about healthy pregnancy...
I don't know, because I can't find one either. I do know that calculating from the fist day of your last period is NOT accurate especially if you practice fertility awareness. The first day of my...