The fetal doppler comes with a 2Mhz probe for fetal detection. This fetal doppler can be used from 12 weeks to measure the fetal heart rate. An interchangeable probe 4 Mhz and 8 Mhz Ensure that these doppler can be used as a vascular doppler. The doppler detects which sensor is connected The interchangeable vascular probe 4 Mhz and 8 Mhz for vascular use is available separately