For expectant mothers like Thanh Tam, who had an 8-week ultrasound and did not hear her baby's heartbeat, it is important to understand that listening to the fetal heartbeat before 10 weeks of gestation is generally not recommended. This early stage is crucial for fetal organ development, making the embryo highly sensitive to external impacts, particularly from certain types of ultrasound.

Typically, before 10 weeks, doctors use 2D ultrasound to confirm the gestational sac's location, identify the fetal heart, count fetuses, estimate gestational age, and calculate the due date. The sound waves in 2D ultrasound have low energy density, do not generate heat, and disperse evenly, making this basic method safe for fetal development.

In contrast, hearing the fetal heartbeat requires pulsed doppler ultrasound. Doppler waves concentrate significant energy and heat into a narrow area, such as the fetal heart, to amplify the sound. This high energy convergence on a fixed point can increase the temperature of the surrounding tissues. Before 10 weeks, when the embryo's organ structures are forming, elevated temperatures can adversely affect this delicate process.

Doctor Long performs an ultrasound for a pregnant woman. Illustration: Tam Anh General Clinic District 7.

Leading medical organizations, including the International Society of Ultrasound in Obstetrics and Gynecology (ISUOG) and the American Institute of Ultrasound in Medicine (AIUM), advise against using pulsed doppler ultrasound to listen for the fetal heartbeat before 10 weeks due to concerns about heat effects on the fetus. Instead, the fetal heart rate can be assessed using M-mode ultrasound, which displays images without emitting sound signals, offering a safer alternative.

Expectant mothers should only undergo doppler ultrasound during the first trimester if there is a suspicion of pathology. For normal pregnancies, doctors rely on 2D and M-mode ultrasound to check the heart rate, ensuring the most scientific protection for the fetus. From the 12th week onwards, when the fetus is larger and more developed, listening to the fetal heartbeat using doppler ultrasound becomes safer.

Doctor Nguyen Hoang Long

Head of Fetal Medicine Unit

Tam Anh General Clinic District 7