4. 1st Trimester Obstetric/OB Ultrasound Protocol

Cardiac Activity (Fetal Heart Rate Measurement)

Assessing cardiac activity is a crucial task in obstetric ultrasound. Fetal cardiac activity should be seen at a crown rump length (CRL) of 5-7mm.

M-mode is the preferred way to measure fetal cardiac activity over pulsed wave doppler because it subjects the fetus to lower ultrasound energy.

  • Step 1 – Locate the fetal heart and optimize the depth
  • Step 2 – Properly place the M-Mode cursor over the beating heart and activate M-Mode
  • Step 3 – Open the OB calculation package and place the vertical cursors at one heartbeat cycle (some machines measure at 2 heartbeat cycles)
  • Step 4– The machine should automatically calculate the fetal heart rate from your measurement.
Fetal heart rate measurement FHR OB Obstetric Obstetrical Ultrasound
Proper caliper placement and fetal heart rate measurement

Normal fetal heart rate ranges:

6-7 weeks100-120 bpm
8 weeks145-170 bpm
9+ weeks120-160 bpm

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