Lola, I went along for my u/s at which time I thought I was 6 - 7 weeks, but I was actually 5 wks and 6 days (according to the length of the bubba). AF had only just become regular - but I had no idea when I o'd.
The u/s lady did detect a heartbeat and my untrained eye could only just see the heart flickering. But the u/s lady did also say that it's not too often they pick up a heart beat that early....so every bubba is different...

As hard as I can imagine it is - try to keep your chin up and think positive thoughts!! Especially with your 20 mth old over Xmas chic!