4+5 is probably too early to see a heartbeat. I thought I was 8 weeks when I went for a scan and there was no heartbeat so they have said I might be just 6 weeks - and 6 weeks is often too early. That or my pregnancy isn't viable. Save yourself the two weeks of torture I'm going through and wait a while before having a scan! Congratulations
I really don't think they are very accurate. They were wildly out for me but I know when I ovulated and the scan also agreed with me. The fact is the stats are based on first wee of the day - which is based on not weeing during the night. How many women in early pregnancy are getting through the night without a wee? Within days of conception my nighttime wee trips had multiplied! I really don't recommend them - they just breed paranoia imho!