At about 18 to 20 weeks, it is possible to hear a baby's heartbeat with a stethoscope, states Health & Parenting. Typically a doctor uses a heart rate Doppler to listen to a fetal heartbeat, which can be heard much earlier than with the use of a stethoscope.
When searching for the baby's heartbeat, it is possible to hear the mother's heartbeat as well as the baby's, states Health & Parenting. The baby's heartbeat is much faster than the mother's, averaging between 120 and 160 beats per minute. Inability to hear the heartbeat should not cause concern as the position of the placenta, age of the fetus, amount of amniotic fluid, weight of the mother, and position of the baby can affect the ability to hear the heartbeat using a stethoscope.