Foetal doppler ultrasound. Hand holding the transducer of a portable foetal doppler ultrasound scanner. The transducer is held against a pregnant woman's abdomen where it emits high frequency sound waves. If the waves hit a moving object the frequency of their echoes will be altered. This allows the speed of blood flow or the foetal heart rate to be measured. Doppler ultrasound is used routinely during antenatal check-ups. This is a Fetal Dopplex scanner.