We often talk about PoCUS for first-trimester bleeding. Is there an IUP? NDIUP? IDK? Yet one of the most common questions patients ask is; “Can you see the heartbeat?” While measuring fetal heart rate is not always our top priority, I’m here to convince you of the following:
- Measuring it still has clinical value.
- It’s easy to do.
Clinical Value
Fetal heart rate (FHR) can be measured as early as 7 weeks transabdominally.1 In the 1st trimester, and for the exam cohort, refer to Box 173.1 (page 2241).2
Ultrasound criteria for an abnormal pregnancy include an absent FHR after 10-12 weeks and a 5-mm crown-rump length without FHR. An absent heartbeat in these scenarios suggests a non-viable pregnancy, such as a miscarriage or anembryonic pregnancy (blighted ovum). However, this should always be confirmed through a radiology-performed transvaginal ultrasound (TVUS) as a premature diagnosis can have significant stress for the expecting patient.3
A low FHR, particularly <90 bpm, is associated with miscarriage.3
In the 2nd and 3rd trimesters, a normal FHR (110–160 bpm) can indicate a healthy fetus.4 Abnormalities in FHR can suggest fetal distress, infection, anemia, or arrhythmias. However, POCUS is only a screening test in this scenario: a single abnormal FHR should prompt you towards confirmatory testing with an NST, radiology-performed ultrasound, or obstetrical consultation.
Measuring Fetal Heart Rate
- Select your curvilinear probe and locate the fetus. It should ideally be done on an “Obstetrical” preset to minimize potential effects on the fetus.5
- Identify the fetal abdomen, often, you’ll see the heart beating in the center of the fetus.
- Resist the urge to use color or pulse wave Doppler. It will not quantitatively measure FHR and exposes the fetus to significantly higher levels of energy compared to M-mode.6 Patients may like to “hear” the heart beating on PW Doppler, but skip the theatrics and just show the images, no sounds.
- Place the M-mode cursor over the beating heart and active M-mode. After a few seconds, freeze the image. You’ll see tiny peaks at the level of the heart.
- Place measurement calipers between two consecutive peaks to calculate FHR. Note that most ultrasound machines have a built-in calculation package under the obstetrical setting. Simply hit “Calc,” select the FHR drop-down box, measure between the two peaks, and voila, it gives you the FHR.
- Manual FHR measurement: If the “Calc” package is unavailable, simply measure the time between two peaks, essentially the R-R interval. Divide 60 by this value to obtain the FHR. For example, if you measured 0.5 seconds: 60 / 0.5 = 120 bpm.
Congratulations, you’ve just measured fetal heart rate!
References
- Levi, C. S., Lyons, E. A., & Lindsay, D. J. (1996). Early diagnosis of nonviable pregnancy with endovaginal US. Radiology, 198(3), 741-745. https://doi.org/10.1148/radiology.198.3.8694073
- Walls, R. M., Hockberger, R. S., & Gausche-Hill, M. (Eds.). (2022). Rosen’s emergency medicine: Concepts and clinical practice (10th ed.). Elsevier.
- American College of Obstetricians and Gynecologists. (2018). Practice Bulletin No. 200: Early pregnancy loss. Obstetrics & Gynecology, 132(5), e197-e207. https://www.acog.org/clinical/clinical-guidance/practice- bulletin/articles/2018/11/early-pregnancy-loss
- Bottomley, C., & Bourne, T. (2009). Diagnosing miscarriage. PeerJ, 1, e82. https://doi.org/10.7717/peerj.82
- Miller, D. L., Abo, A., Abramowicz, J. S., Bigelow, T. A., Dalecki, D., Dickman, E., Donlon, J., Harris, G., & Nomura, J. (2020). Diagnostic ultrasound safety review for point-of-care ultrasound practitioners. Journal of Ultrasound in Medicine, 39(6), 1069-1084. https://doi.org/10.1002/jum.15202
- Salvesen, K. Å., & Lees, C. (2011). ISUOG statement on the safe use of Doppler in the 11 to 13+6-week fetal ultrasound examination. Ultrasound in Obstetrics & Gynecology, 37(5), 625-628. https://doi.org/10.1002/uog.9026
For images:
- https://www.researchgate.net/figure/Fetal-heart-exposed-outside-the-thoracic-cavity-of-Twin-B-at-9-weeks-gestation-The_fig1_282041764
- http://christem.com/11-vaginal-bleeding
- https://www.usa.philips.com/healthcare/product/HC795090CC/sparq-ultrasound-system
