What is the difference between a 2MHz, 3MHz, 4MHz, and 8MHz probes?

The 2MHz and 3MHZ probes are used for the detection of foetal heart rate from early gestation through to delivery. These probes are provide a general indication of foetal wellbeing and can be used to verify foetal heart viability following patient trauma.

The 4MHZ and 8MHZ probes are used to indicate the blood flow in veins and arteries for assisting in the detection of peripheral vascular disease.