How far along in pregnancy are you after your first missed period?

Well, that is something that an ultrasound will have to determine. The theory is that you are pregnant (conception has occurred) sometime after your last period. Conception doesn't necessarily occur the moment that the man ejaculates his sperm into the vagina. So, yes, you did become pregnant somewhere between when you had sex after your last period, but before you realized that you were late in getting your period. This has to be discovered by the ultrasound, to determine the EDD (estimated due date).