How to correctly determine the date of conception?

How to correctly determine the date of conception? According to one screening, my dates according to menstrual cycles and ultrasound coincided day by day, the LMP was August 27th and I calculated the conception date online with an electronic calculator on December 4th. According to the second screening, the LMP was set for August 23rd, now according to ultrasound (baby's size), the due date is 6 days later, the LMP shifted even further. Should I rely on the first screening to determine the exact date of conception?