heartbeat generally doesn't start until fetus is 5w3 or 4d - so LSK's scan at 5w might have been a bit later but baby measuring small. they're so tiny at that point, that they tend to have up to a one week window of error

as a rule, most places wont' scan until at least 7 weeks as the baby isn't big enough to see anything on an external scan at that time. i know of at least one BB-ite who was told they had a 5w sack, come back in 2weeks to check viability. it's better to wait a bit longer...