HCG levels after u/s didnt show hb

I had an u/s yesterday at 6w 3d which showed a 5 1/2 week gestational sac and 6 w yolk sac but no sign of baby or hb. I got hassled coz I had a m/c in Feb so went in for a blood test and my level has gone up to 24,500 on 15th May from 3,500 on 9th May. I had heard levels are supposed to atleast double every 48 hrs so I should have been 28,000? Am I OK if its just below that? I have my next u/s on the 28th so just need reassurance that these levels look OK.
Thanks so much to all who reply...

[L J]

I found an hcg calculator that may put your mind at ease:

http://www.justmommies.com/articles/hcg-levels.shtml

[L J]

Just to let you know that levels double every 2 to 3 days not every other day, so your levels are perfectly normal.

[Swats]

Thanks...that does make me feel better!

[rubyolivia]

My levels never doubled....just increased. It wasn't until 7 weeks that I saw fetal pole and heartbeat! I would say everything is fine! Seems like there are an awful lot of posts about this subject, so it must be fairly common. Yet our darn doctors don't really act like it is...can't figure that one out! Good Luck! :)