The date given to you by your doctor is the most exact that you will get unless you know the exact dates that you had sex and they were separate enough to be able to know on your own when you conceived. The doctor takes into account the size of the fetus on the ultrasound, your last menstrual period, and your estimated ovulation dates to get a conception date. If you still want a second opinion, I input your information into a pregnancy calculator using the date of your last menstrual period and got a conception date of April 24.