When Can You Hear the Baby's Heartbeat on a Doppler?

A fetal Doppler is a handheld ultrasound device that looks like a small radio with a transducer attached. Doctors use it to listen to a yet-to-be-born baby's heartbeat. For most women, their baby's heartbeat should be detectable via Doppler by 12 weeks of pregnancy -- the end of the first trimester.

However, there's a lot of individual variation in when the heart will become detectable. Some women may hear the heartbeat with a Doppler device as early as 8 weeks, while others may not hear it until closer to 12 weeks. For this reason, some doctors won't even start checking for the heartbeat with a Doppler device until women are 12 weeks pregnant.

If you're contemplating renting a Doppler device for home use, it might be best to wait until your doctor has found the heartbeat at one of your prenatal checkups so you know for sure that it should be detectable (and you can ask for tips on how to find the heartbeat with the home device).

Listening for the baby's heartbeat can be both exciting and anxiety-inducing if you have a history of miscarriage. Hearing the heartbeat is a great reassurance, but not detecting it might make you feel nervous -- even though it doesn't necessarily mean anything's wrong. Remember to keep the latter point in mind if you're checking for the heartbeat at home and you're not yet 12 weeks along.

Also note that once you are in the third trimester, you should pay attention to your baby's movement patterns and contact your doctor if you notice a reduction in movement. Don't rely on a heartbeat monitor to judge whether your baby is OK.

The Food and Drug Administration (FDA) has issued a warning for expectant mothers to use handheld Doppler devices at home with caution. Handheld Doppler devices are legally marketed as "prescription devices," and, according to the FDA, "should only be used by, or under the supervision of, a health care professional."

While there is no evidence that Doppler devices cause harm to a developing baby, ultrasound can slightly heat tissues. "When the product is purchased over the counter and used without consultation with a healthcare professional taking care of the pregnant woman, there is no oversight of how the device is used. Also, there is little or no medical benefit expected from the exposure," said Shahram Vaezy, Ph.D., an FDA biomedical engineer. "Furthermore, the number of sessions or the length of a session in scanning a fetus is uncontrolled, and that increases the potential for harm to the fetus and eventually the mother."

In addition, there is concern that in the untrained hands of pregnant mothers who are not trained medical personnel, home Dopplers can give a false sense of reassurance: Upon hearing what a woman believes to be a heartbeat, she may delay seeking medical treatment when it is actually warranted.