Please dont buy one, they are crap and dangerous really i think.
I attempted to use one of these on someone few yrs ago as she was trying unsuccessfully to hear anything.
Now, i am actually a midwife and i failed to pick up anything!! I told her to get a refund.
While its absolutely lovely to hear the heartbeat, even if you do, you will still not be aware if there is a problem, only that the baby is 'alive' if you get me. use other signs to check babies wellbeing instead.
At 19 wks, movements will not always be regular or that noticeable, but in a few wks i promise they will be and then use the regular pattern to see if baby is moving as much as normal for you. At this stage you will often be unsure of what you are feeling. If you are worried about anything, see the midwife, she will easily get the heartbeat at your gestation.
If its any consolation i was a bit mithered that my ds2 was ok when i was only a few wks preg. There wasnt a problem or anything, it was before i was able to feel movements and i was curious. I was probably 13 or 14 wks but cos i have a sonicaid i could hear the heartbeat very early on at home! However, once i got regular movemnts i never ever bothered listening at home cos my movements were so obvious i didnt need to hear to know he was ok! You will be the the same in a few wks