What is the conception date if the ultrasound today says the fetus is 12 weeks and 6 days old?

To determine the conception date based on a fetus being 12 weeks and 6 days old, you would subtract that gestational age from today's date. Since gestational age is typically calculated from the first day of the last menstrual period (LMP), you would subtract approximately 13 weeks (or 3 months and 6 days) from today's date to estimate the conception date. This would mean the conception likely occurred around 3 months and 6 days before today.