When to have a scan

Kerry
I'm currently 4 wks pregnant and want to go for an early scan . I had a m/c in august at 12 wks but the baby hadn't grown past 5 wks . So this time I feel a bit anxious as I don't won't to get that far into the pregnancy to only be told the same thing again. So really I'm wondering when does the babies heartbeat become visable ?