Hi dan-o,
I have the same thing, levels are taking a bit longer to double, but i had a scan this morning and we saw a little yolk sac (we couldn't see anything this time last week) i am only 5 weeks. Everything is measuring bang on though, and midwife said not to worry too much about slowish doubling time, that as long as they are rising within 72 hours she is happy with that. Are you going to have an ultrasound?
Try not worry too much, though that is easier said than done!