Doppler studies use the same principle that police speed radar guns operate on.
It is able to detect the speed and direction of movement. This is expressed as red and blue colours on the scan.
This is useful in measuring blood flowing in the placenta, umbilical artery and baby's heart.
These figures are then compared against the normal values to make sure all is progressing well.
If there is a problem it is not unusual to have this scan every few days.
Baby's heart showing the 4 chambers and Dopplers
Baby's Umbilical Chord and Dopplers