Tell them you aren't sure when your last period was and ask them to tell you the estimated due date of the baby, based on the embryo's measurements and developmental markers. You're still early enough in the pregnancy
for an ultrasound to be helpful in dating conception
, but this won't be true for long. When they give you an estimated due date, go home and plug it into an online conception calculator or just count back from it on a calendar 266 days.