I Had An IUI Done On Dec 30th, And Had

I had an IUI done on Dec 30th, and had my first u/s done today (Feb 1). They are saying I am 6 weeks and 4 days, which doesn't make sense. The way I count it, the fetus is 4 weeks and 5 days of conception occurred the day of the IUI? The u/s only showed a gestational sac. No baby, no yolk sac, no heartbeat. According to the doctor the pregnancy isn't viable. I'm confused on caculating fetal age via an IUI? Can someone please explain and also should I be seeing more on the u/s at this point?