What is a Doppler Fetal Monitor?

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  • Written By: Janis Adams
  • Edited By: C. Wilborn
  • Last Modified Date: 13 March 2020
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A Doppler fetal monitor or Doppler fetal heart rate monitor is a device commonly used during prenatal examinations to translate the sounds of a baby's heartbeat, while still in utero, to an audible level. The device employs the Doppler effect to produce an audible heart beat simulation. Using a Doppler fetal monitor is a non-invasive diagnostic procedure employed during routine prenatal care, and is generally considered safe.

First documented by Christian Doppler, the Doppler effect translates the changes in the frequency of a sound wave emitted by a source when moving in relation to an observer. Therefore, the Doppler fetal heart rate monitor detects the high frequency sound waves emitted when a baby's heart beats. When a heartbeat is heard with Doppler, it is not the actual heart beat but a representation. Use of this device is often referred to as Doppler auscultation.

To conduct Doppler fetal monitoring, a gel or oil solution is spread liberally across the pregnant mother's belly. This solution acts as a conductor. A probe or transducer is then traced back and forth across the belly until the heartbeat is successfully detected. Once the sound waves emitted by the fetus' heart are detected, they are transmitted to the monitor where the sound of them is then amplified to an audible level.


There are advantages of the Doppler fetal monitor over a fetal stethoscope, which also detects a baby's heartbeat when in utero. The Doppler fetal monitor permits everyone present in the room at the time of the examination to hear the heart beat of the baby. The fetal stethoscope only affords the person wearing the earbuds connected to the device the opportunity to hear the fetal heart beat.

Doppler fetal monitors traditionally come in one of two frequency levels, 2 or 3 MHz probes. To detect a fetus's heart rate in the earliest weeks of pregnancy — from 8 to 10 weeks into the gestational period — the 2 MHz device is commonly the one of choice. For those women who have been determined to be overweight state during pregnancy, however, the use of the 3 MHz device is more commonly suggested.

Many women choose to rent or buy Doppler fetal monitors during their pregnancy, as they like to be afforded the opportunity to hear or check their baby's heart rate at any given time. Created for home use, these models most often run on batteries. Some models made for home use are digital and offer the added convenience of a display screen, which shows the number of heartbeats per minute.


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