Official dates put me at 4 weeks 1 day pregnant today. If you're paying attention you will realize that since retrieval was 15 days ago the embryo(s) can't possibly be any older than that. It's just the old-fashioned way of measuring these things. Back when people just got pregnant from 'making the beast with two backs' as Shakespeare put it, women had no clue when the ovulated, so they just counted from the first day of the last period. Silly, but that is where those two extra weeks come from.
I go back in for another test on Monday. That's the next big hurdle. We want that number to double every 36 to 48 hours. If the number is appropriately higher on Monday that's a good sign. If not... well, I'll worry about that later.
Oh, and the normal hCG range for this point is 50-150, so 278 is quite nice. It could mean that there is more than one in there, or it could mean that there is one really kick-ass embryo in there. It could just be a statistical blip. There's no way to tell until the first ultrasound.