Fingers crossed for you Libby, try not to worry.
I had a scan last time at what I thought was 6 wks & I was only measuring 5 so all they saw was a sac, a week & a half later I saw a heartbeat. Unfortunately that pregnancy did end in miscarriage for me.
5 wks can just be too early to see anything so try to stay positive & hopefully you will have 2 little heartbeats to talk about in a week or 2.
xx