If not, it is going to be very difficult to know whether you got pregnant from
sex on the 23rd or the 28th, since
after a while in pregnancy there is a margin for error when using an ultrasound to try to determine conception. It would be hard enough to split the difference between the 23rd and the 28th without that problem, but if your ultrasound was not until after your first trimester was over, then it could be giving you a date as much as a week off.