Honestly, most of us waited until around 12-14 weeks before trying the doppler kasi mas malinaw na ang heartbeat by then, though some moms start as early as 10 weeks if they're excited (haha!). The thing is, if you can't find it, don't stress yourself out, it's totally normal, especially in the beginning, kaya siguro better to ask your OB first kung kelan okay lang gamitin or let them do it para walang alalahanin!