How accurate is an ultrasound done at 13 weeks, for dating a pregnancy? if exact conception dates are known can 13wks actually be 15wks 6 days!

Quite possible!: First your ega ("how far along") is 2 weeks longer than actually conceived, because it's dated from a theoretical last period, not conception. There's + 2 extra weeks. Second, us in 2nd trimester (13 weeks) is accurate to about 1 week, so there's +/- another week. Third (politely) you may know the exact day you had sex, but you don't know the exact day you conceived unless you did ivf.