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.
Normal fetal heart rate ranges:
| 6-7 weeks | 100-120 bpm |
| 8 weeks | 145-170 bpm |
| 9+ weeks | 120-160 bpm |