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Fetal Dopplers1

What is fetal doppler?
A fetal doppler is one type of fetal monitoring. The fetal heart doppler is a non-invasive diagnostic instrument used to detect and measure the fetal heart rate. A fetal heart doppler measures the fetal heart rate by emitting and receiving continuous ultrasonic sound waves and emitting the change or shift in pitch frequency of these sound waves. This is called the Doppler Effect and was discovered by Austrian mathematician and physicist, Christian Doppler (1803-53). Each fetal heart monitor uses a transducer or probe of varying MHz (millions of cycles per second), usually 2 or 3, containing piezoelectric crystals which send short sound pulses into the directed area of the body. Every time one of these sound waves is sent the transducer (probe) pauses briefly and listens for the returning echo at which time the fetal heart doppler determines the depth and direction of each returning sound wave and converts the signal into the fetal heartbeat you hear. This heartbeat is not your baby's actual heartbeat but a reproduction.

How do I use my fetal doppler?
The best way to begin an exam is to be lying down exposing the abdomen, although it may be possible to find the heart rate in a sitting or standing position. Apply a generous amount of ultrasound gel to the fetal doppler probe. Early in pregnancy the best technique is to begin with the transducer (probe) on the midline of the abdomen (above the pubic line) aiming downward behind the pubic bone. The probe should then be angled slowly in search for the fetal heart rate. Be patient, early in pregnancy the size of the fetal heart is so small and the sound so faint it can be very easy to pass over and miss the sounds. A 2 MHz probe is best for early detection. A full bladder can make it easier to find the heartbeat. Try to keep from dragging the probe around as it will only increase static and make it more difficult to find the heartbeat.

When can I start using a fetal doppler?
In some woman the fetal heart can be detected as early as 8-12 weeks. The fetal heartbeat should be audible 95% of the time in 12-week pregnancies, and nearly 100% of the time after 14 weeks. Earlier in pregnancy, between 8-12 weeks, your success in finding the heart rate can have to do with the quality of your device. Fetal dopplers that employ a 2 MHz probe tend to pick up the fetal heart rate earlier than 3 MHz fetal dopplers as they have a more defined beam which penetrates deeper (3-5cm compared to 1-2cm). An important note is to remember that everyone is different, so not finding your baby's heartbeat early isn't a reason for concern. If you have concerns about your pregnancy you should contact your healthcare provider. Like the numbers above reflect nearly 100% of woman are able to hear the fetal heartbeat from the 14th week.

How often can I use my fetal doppler?
The Food and Drug Administration regulates ultrasound in the United States and has cleared it for "continuous use". As there is no consensus on what this means or how often you should use your fetal doppler it is commonly recommended that like everything in life prudence is the best policy. When using your fetal doppler it is suggested that you use it in short intervals, infrequently, and following the first trimester.

As a general rule an exam with a fetal doppler should never last longer than 10 minutes. If you don't find the heartbeat in this period of time, take a break. If your concerned contact your health care provider. If not, try again in a few days.

The AIUM (American Institute of Ultrasound in Medicine) suggests "prudent" use.

How do I know I'm hearing my baby's heartbeat and not mine?
The easiest and most conclusive way to decipher your heartbeat from your baby's is by the heart rate or BPM (beats per minute). Your baby's heart rate will be between 120 & 180 BPM. This is considered the normal range of a fetal heart rate. Conversely, your heart rate will be much less 60-100 BPM. Early in pregnancy its normal for the fetal heart rate to vary within a reasonable range. As your pregnancy progresses and your baby grows the heart rate will become more static. Also be aware that like adults your baby's heart rate will vary depending on activity level. If you have further questions or concerns about your baby's heart rate contact your health care provide.

How do I calculate my baby's fetal heart-rate?
To calculate your unborn baby's fetal heart rate simply count the beats you hear for 15 seconds and multiply that number times 4. This will give you an average of your baby's (BPM) beats per minute (15 seconds multiplied by 4 equals 60 seconds). You can count for any interval of time (i.e. 10secs x 6, 20secs x 3, 30secs x 2) and multiple that number to get to 60secs (1minute).

What should my baby's fetal heart-rate be?
The normal fetal heart rate is between 110 to 180 beats per minutes (BPM), but can vary. Fluctuations of the fetal heart rate (usually associated with fetal movement) during different periods of the day are common and often rise above 160, going as high as 180 to 190 and are considered normal. If you have questions or concerns about your baby's fetal heart rate always direct them to your healthcare professional.

What hearing a healthy heartbeat signifies
Listening to a normally beating heart using a fetal ultrasound doppler as early as 8 weeks can offer reassurance and cut down on a lot of stress and help assure that the developing fetus is healthy. While miscarriage occurs in only about 15 percent of apparently normal pregnancies, it only occurs in about 1 percent of pregnancies where a normal heartbeat has been seen or heard.

Can stress affect the fetal heart rate?
Recent studies conclude that changes in pregnant women's heart rate and blood pressure due to chronic stress and anxiety, can have an affect on the fetal heart rate.The study published in Developmental and Behavioral Pediatrics , February 2003 did not report any negative effects on fetal health but confirmed that emotional based changes in a woman's cardiovascular activity can have "real-time effects on a fetus according to Catherine Monk, Ph.D.

Previous study has shown that stress during pregnancy can cause an increase in the risk of low-birth weight and premature birth. More importantly, increased evidence suggests that pregnancy stress can actually affect the baby's behavior and functioning later in life.

The ability for expectant mothers to listen to the fetal heart rate with a fetal doppler offers a safe method of early fetal bonding which was previously only available at prenatal appointments with a medical professional. With its approval for in home by the FDA expecting parents have increasingly been opting to rent or purchase fetal dopplers to help relieve some of the stress associated with the unknowns of early and late pregnancy. Fetal doppler rentals are currently available through many online retailers at very affordable rates.

Can the fetal heart rate predict gender?
This old wive's tale just doesn't want to die. Your baby's fetal heart rate has nothing to do with the sex of your baby, though many have a great time with this myth. What you'll find as your baby grows and gestation progresses is that the heart rate will actually become slower. Does this mean we all begin life as a female? Genetically yes, but the heart rate still has nothing to do with the sex of your baby. Stick to the old fashioned way of finding out the sex - birth!

How does Fetal heart beat sounds?

Fetal Sounds
A galloping horse is what your baby's heartbeat will sound like and will be about twice as fast as the mother's heart rate. An average normal fetal heart rate is between 120-180 BPM (Beats Per Minute). Where as an average adult heart rate at rest is between 60-100 BPM.

Maternal Sounds
You may also hear sounds of your own heart or blood flow. A major artery runs through your abdomen and the swish of your blood through it is sometimes picked up by the fetal doppler. This sound is heard by the baby later in pregnancy and has been recorded and used to calm upset babies after birth.

Placental Sounds
The blood flow becomes steadier as it flows through the placenta and has a distinctive sound like wind blowing through the trees.

Instrument Noise
The fetal doppler itself may have some background noise, static, popping etc. When the fetal doppler probe is moved on the abdomen, louder popping noises may result. Sometimes, the movements of your baby in later pregnancy also produce louder noises. Ultrasound gel use is integral on cutting down on much of the static produced when positioning the fetal dopplers transducer during an exam.

Differences between 2 MHz and 3 MHz Fetal Doppler probe

All baby heart monitors designed for fetal heartbeat monitoring is equipped with either a 2 or 3 MHz probe. With a 2 MHz probe the beam is narrow and direct which helps it penetrate deeper (3-5cm compared to 1-2cm) and find the fetal heartbeat earlier than 3 MHz dopplers.

  • The higher the frequency, the shorter in duration is the period and the shorter is the wavelength
  • The lower the frequency, the longer in duration is the period and the longer is the wavelength
There is a lot of information available stating that the 3 MHz fetal doppler probe actually allows the user to find the heartbeat much easier and earlier. We disagree. In our tests and review we found that a 2 MHz doppler probe picked up the fetal heart rate at 8 weeks 80% more of the time than a 3 MHz probe. Part of the argument for the 3 MHz probe enabling easier detection of the heartbeat comes from the belief that because the higher frequency probe has a wider beam the heartbeat is easier to find. When in fact after 12 weeks gestation either probe is easy to find the heartbeat with. This and the fact that a 2 MHz probe is much more useful for overweight woman is why we recommend the 2 MHz probe and feel it's a better all around probe.

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