Understanding Fetal Heart Rates and Doppler Sonography

Hearing your baby’s heartbeat for the first time is one of the most special moments of any pregnancy. Although, as you hear it at every office appointment after, it never becomes any less special. Many parents have general questions about the process of fetal heart rate detection and become concerned at what impact imaging technology can have on their developing baby. Here’s a look at how doctors are able to hear and see a developing fetal heart beat. 

How does a Doppler sonography wand work?

The Doppler sonography instrument is a small, handheld, wand-like device. The Doppler wand works by using sound waves are bounced to blood vessels and the baby's heart and then back to the device. In the first trimester, Doppler wands may be used vaginally. The uterus and enclosed fetus are still snug within the pelvic area and covered in a layer of fat -- no matter how thin you are. A vaginal wand lets the technology get closer to a very small target.
Later, your doctor will probably put a small amount of gel to the wand or your abdomen and apply a little bit of pressure to your abdomen just above the pubic bone. It may take a little bit of sliding around to find your growing baby, so don’t be alarmed if it takes a few minutes. 

When does your baby’s heart become visible and audible?

About six weeks after conception, a growing baby becomes developed enough that an ultrasound can visually pick up it’s fluttering heartbeat. Seeing that movement can be reassuring for new parents -- not to mention exciting. By the end of the third month, the heartbeat is generally strong enough for a Doppler sonography device to pick up and project the baby's heart rate, allowing your first audio contact with your baby.

How do doctors hear fetal heartbeats? 

Ultrasound technology bounce sound waves off your baby to project an image in realtime -- this allows a doctor to monitor the growth and development of your baby. The soundwaves of your baby's heartbeat are amplified through a speaker attached to the Doppler. An average fetal heart rate generally ranges between 120-160 beats per minute. 

What if I can’t hear my baby’s heartbeat?

Remember, at this point your bundle of joy is about the size of a plum and their heart considerably smaller. Aside from size, all the layers of tissue and amniotic fluid between wand and fetus can hinder audio in the early months. Additionally, if your baby likes to slide into abnormal, out-of-the-way positions, your doctor may even have trouble getting a heartbeat. That doesn’t mean anything is wrong, just mean that your baby is tricky to pinpoint.

What happens when Doppler sonography doesn't work?

If your doctor can’t detect a heartbeat in the early months when using Doppler sonography, they’ll likely rely on ultrasound imaging. Through an ultrasound, your doctor (and you) will be able to see the heartbeat, even if you can’t hear it. Because of the difficulty of locating a heartbeat, and the inherent panic associated with not hearing one, most experts caution against buying an at-home Doppler instrument. Instead, they suggest parents rely on the experts and their infinitely more expensive and accurate machines. 
Last Updated: November 13, 2017