usually hcg levels have to be above (opinion seems to vary on this) at the LEAST 2500-5000miu before you can see the heartbeat and a fully developed
fetal pole...so I would say in the next
week or two you should be able to get a clearer picture and a more thorough measurement. I'm not a doc, just going by experience, but that would be my guess :-). the 7w2d is probably fairly accurate (maybe a miscarriage followed by an immediate conception, like you've thought before?