You midwife should check it at every appointment. It's better to just let them do it then and only use it when you need to.
Ive had mine since before 20 weeks (im now 33) and ive only used it to show family members like 5 times all together, and 1 time because I was scared and maybe 6 times when I was really depressed and crying and needed to hear it.
The baby is definitely effected by it, like he or she can tell there's somwthing there, but it isnt very hurtful UNLESS you continued to use it every day multiple times.
Now that my baby is bigger I rely on being able to feel him move over his heartbeat, cuz even though hes 33 weeks sometimes it's still hard to find his heartbeat cuz he moves so much.
Well Doppler's and ultrasounds are sound waves so it's like standing next to a train eveytime you do it . idk how it effets the baby in the long run but I'd recommend the last three better