When a Few Degrees Change the Diagnosis
In obstetric ultrasound, Doppler is not decorative.
It guides clinical decisions.
A small technical mistake can lead to a big clinical conclusion.
1️⃣ Umbilical Artery (UA)
What We Measure
- S/D ratio
- PI (Pulsatility Index)
- End-diastolic flow
Common Technical Pitfalls
✔ Angle not aligned with flow
✔ Sampling too close to placental insertion
✔ Excessive probe pressure
✔ High wall filter removing diastolic flow
If angle correction is ignored → velocity underestimated
If wall filter too high → false absent diastolic flow
Always sample from a free loop of the cord.
2️⃣ Middle Cerebral Artery (MCA)
What We Measure
- Peak Systolic Velocity (PSV)
- PI
Used for:
- Fetal anemia screening
- Brain-sparing effect
Critical Technical Points
✔ Near 0° angle when possible
✔ Sample at proximal MCA
✔ Avoid oblique section
Even small angle error changes PSV significantly.
MCA PSV is angle-sensitive. Small misalignment can mimic anemia.
3️⃣ Uterine Artery (UtA)
What We Evaluate
- PI
- Early diastolic notch
Used for:
- Preeclampsia risk
- Placental insufficiency screening
Common Errors
✔ Incorrect vessel identification
✔ Measuring too distal
✔ Improper angle correction
Notch appearance may change with gain and filter settings.
Confirm vessel identity before trusting the waveform.
Quick Comparison Table
| Vessel | Key Parameter | Most Sensitive To |
| UA | S/D, PI | Wall filter & sample site |
| MCA | PSV | Angle alignment |
| UtA | PI, notch | Vessel identification |
Practical OB Reminder
If a Doppler value looks abnormal:
- Check the angle.
- Re-align the cursor.
- Confirm the vessel.
- Repeat measurement.
Do not diagnose from one waveform.
Sonographer’s Note
In obstetrics, Doppler numbers carry weight.
They influence monitoring, admission, even delivery timing.
But before reacting to a number,
I ask myself:
Did the physics agree with the anatomy?
Because sometimes,
a few degrees are louder than pathology.
