The Bistos BT-220 Fetal Doppler accurately measures fetal heart rate and outputs fetal sound through a built-in speaker that produces a clean, clear sound. The BT-220 offers an ergonomic design, fetal acoustic stimulator, 2.4 in. large color TFT LCD display with a heart rhythm indicator. Additionally, the high-sensitivity doppler 2 or 3 MHZ probe is waterproof and uses 2 AA batteries that last about 360 minutes.