Before the first 20 weeks of pregnancy you'll be unable to hear the babies heartbeat with a stethoscope and to hear the babies heartbeat before the first 20 weeks of pregnancy you'll require a heart rate Doppler.
After 20 weeks of pregnancy then a stethoscope can be used to hear the babies heartbeat without that fancy heart rate Doppler.
So as long as you're more than 20 weeks pregnant you should be able to hear the baby and the babies heartbeat with a normal stethoscope.
You can rent or buy a heart rate Doppler to hear the baby before 20 weeks of pregnancy but it's best to just wait until after 20 weeks of pregnancy where a normal Stethoscope will work.