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 find 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 in 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.
ACOG Education Pamphlet AP032. ACOG. Accessed: 16 Jan 2010. http://www.acog.org/publications/patient_education/bp032.cfm