Current evidence suggests that using Doppler ultrasound in high-risk pregnancies to monitor a fetus’ health may reduce caesarean sections and the number of babies who die.
Doppler ultrasound is a noninvasive test that can be used to measure your blood flow and blood pressure by bouncing high-frequency sound waves (ultrasound) off red blood cells. A regular ultrasound uses sound waves to produce images, but can’t show blood flow. A Doppler ultrasound can estimate how fast blood flows by measuring the rate of change in its pitch (frequency). Developments in doppler ultrasound technology in recent years have enabled a great expansion in its application in Obstetrics, particularly in the area of assessing and monitoring the well-being of the fetus, its progression in the face of intrauterine growth restriction, and the diagnosis of cardiac malformations. Doppler ultrasound is presently most widely employed in the detection of fetal cardiac pulsations and pulsations in the various fetal blood vessels. The “Doptone” fetal pulse detector is a commonly used handheld device to detect fetal heartbeat using the same doppler principle. Blood flow characteristics in the fetal blood vessels can be assessed with Doppler ‘flow velocity waveforms’. Diminished flow, particularly in the diastolic phase of a pulse cycle is associated with compromise in the fetus. Various ratios 
of the systolic to diastolic flow are used as a measure of this compromise. The blood vessels commonly interrogated include the umbilical artery, the aorta, the middle cerebral arteries, the uterine arcuate arteries, and the inferior vena cava. The use of color flow mapping can clearly depict the flow of blood in fetal blood vessels in a realtime scan, the direction of the flow being represented by different colors. Color doppler is particularly indispensible in the diagnosis of fetal cardiac and blood vessel defects, and in the assessment of the hemodynamic responses to fetal hypoxia and anemia. A more recent 
development is the Power Doppler (Doppler angiography). It uses amplitude information from doppler signals rather than flow velocity information to visualize slow flow in smaller blood vessels. A color perfusion-like display of a particular organ such as the placenta overlapping on the 2-D image can be very nicely depicted. The power emitted by a doppler device is greater than that used in a conventional 2-D scan. Its use in early pregnancy is therefore cautioned.
The review included 18 studies which together included 10,000 women in “high risk” groups. High risk women included those who had previously lost babies during pregnancy, those carrying growth restricted babies and women with hypertension or diabetes. Women who were examined with Doppler ultrasound were compared with those who had no Doppler or with those who had cardiotocography (CTG), which monitors the baby’s heartbeat. According to the results, Doppler reduced infant deaths, possibly through better timing of caesarean sections, as well as reducing the number of caesarean sections themselves, and inductions of labour. However, the researchers say the studies included were of questionable quality.
“A case could certainly be made for a higher quality, multi-centre trial of Doppler ultrasound than we have so far seen,” said lead researcher Zarko Alfirevic, who is based at the Division of Perinatal and Reproductive Medicine at the University of Liverpool. “It is quite possible that for some so-called high risk groups fetal Doppler offers little or no benefit. Women with diabetes are one such group where fetal Doppler may, in fact, give false reassurance.