Why are fetal heart tones (FHT) checked starting at 16 weeks of gestational age (WGA) when the fetal heart develops at 4 weeks?

From the Guidelines

Fetal heart tones are typically checked starting at 16 weeks of gestational age because, although the fetal heart begins developing around 4 weeks, it is too small to be detected by conventional methods until later in pregnancy. At 16 weeks, the fetus has grown large enough that the heartbeat can be reliably detected using a handheld Doppler device during routine prenatal visits. Before this time, the fetal heart may be visible on ultrasound (as early as 6-7 weeks), but it is difficult to auscultate with standard clinical tools. The normal fetal heart rate ranges from 110-160 beats per minute once detectable.

Key Points

• Fetal heart development begins around 4 weeks, but it is not detectable by conventional methods until later in pregnancy 1.
• At 16 weeks, the fetus has grown large enough that the heartbeat can be reliably detected using a handheld Doppler device during routine prenatal visits.
• Regular monitoring of fetal heart tones is important as it provides reassurance about fetal viability and well-being.
• If fetal heart tones cannot be detected by 16 weeks with a Doppler, further evaluation with ultrasound is usually recommended to confirm fetal cardiac activity and proper development.
• Recent studies, such as those published in 2024, support the practice of starting fetal heart tone checks at 16 weeks of gestation 1.

Recommendations

• Healthcare providers should start checking fetal heart tones at 16 weeks of gestational age.
• Use a handheld Doppler device to detect the fetal heartbeat during routine prenatal visits.
• If the fetal heartbeat cannot be detected, further evaluation with ultrasound should be performed to confirm fetal cardiac activity and proper development.
• Recent guidelines, such as those from the Society for Maternal-Fetal Medicine, recommend routine ultrasound surveillance for twin-twin transfusion syndrome starting at 16 weeks of gestation 1.

From the Research

Fetal Heart Tone Checks

• Fetal heart tones are typically checked starting at 16 weeks of gestational age (WGA) despite the fetal heart developing at 4 weeks 2, 3.
• The reason for this delay is largely due to the limitations of detection methods and the size of the fetus at earlier stages of gestation.

Detection Methods and Limitations

• Studies have shown that fetal heart tones can be detected as early as 17 weeks using an ordinary fetoscope, with detection possible in 81% of patients examined at 20 weeks and virtually all patients examined at 21 weeks or later 2.
• The use of ultrasound equipment has improved the ability to detect fetal heart tones at earlier stages, with reliable measurements possible from 16 weeks' gestation onward 3.
• Advances in technology, such as three-dimensional and four-dimensional ultrasound modalities, have further improved the capabilities of fetal echocardiography screening, allowing for earlier detection of congenital heart defects 4.

Early Detection and Screening

• Early fetal echocardiography can be offered as a screening examination to at-risk and low-risk patients, with the proviso that it be repeated following screen-negative scans at mid-gestation to exclude later developing lesions 4.
• Studies have demonstrated the accuracy of early fetal echo (EFE) in detecting fetal cardiac disease, with sensitivity, specificity, and positive and negative predictive values of 92.9%, 100%, 100%, and 99.7%, respectively 5.
• Fetal cardiac Doppler signal processing techniques have also been developed to improve the detection of fetal heart rate and cardiac valve motions, with recommendations for future research directions to address the underlying challenges 6.