When I went in for my first scan, I was put 9 days behind (but put me at 5w 5d and the heartbeat also was not strong yet). Since then, we've been in limbo because of the heartbeat. I've been looking for stories that show a positive outcome, and really I've seen a ton where baby catches up or stays healthy even when measuring behind. Especially if the heartbeat is strong, at that point it's more unlikely to have problems- but even if the hb is slow, things can turn around then, too.