A fetal Doppler ultrasound is a way of monitoring the development of unborn babies throughout pregnancy. Its specialty is the ability to observe blood flow through the fetus, ensuring that he is alive and thriving, though it should be noted that most birth defects cannot typically be seen with this type of ultrasound. It works by emitting sound waves that create a picture once they hit the blood cells of the fetus. Not only can the doctor and patient hear the blood flowing when a fetal Doppler ultrasound is in use, but they can also typically see it on a screen. Thus, this is one of the most popular types of ultrasounds used in obstetrics.

This device uses the Doppler Effect, which is the change in both the wavelength and frequency due to a moving object, such as blood cells. The typical fetal Doppler ultrasound contains crystals that can release sound waves through the uterus, which usually come back toward the device's transducer after making contact with the unborn baby. This usually creates an echo, which can be measured in terms of its strength, frequency, and the amount of time that passes between its emission and its return. The result is an image on the ultrasound monitor, as well as the sounds of blood flow through the blood vessels, umbilical cord, and heart.

Typically, fetal Doppler ultrasounds are used to ensure that the fetus is both alive and growing as it should. Therefore, this type of diagnostic tool may be used at regular appointments, and also when there is a question about the health of the fetus. For example, if the mother notices decreased movement, her doctor will usually schedule a fetal Doppler ultrasound to ensure that the blood is flowing normally. Blood that is flowing particularly quickly or slowly through the blood vessels, heart, or umbilical cord may signal fetal distress, which is typically important to diagnose and treat immediately.

Aside from normal blood flow, a fetal Doppler ultrasound can also often detect abnormalities in the heart. Of course, in many cases, the two are related, but occasionally, a heart defect may not be made apparent just by glancing at blood flow. In addition, doctors can usually detect growth restriction in the uterus, which may prevent the fetus from developing normally. Thus, this type of ultrasound is often quite helpful in obstetrics, and is also usually considered less harmful than X-rays, contributing to its popularity in this field.