Amniotic fluid volume
Amniotic fluid volume (AFV) is not routinely measured quantitatively as part of the screening. However, qualitative assessment of amniotic fluid is still done, and significant abnormalities may be noted.
Points about AFV during NT Screening
1. Amniotic Fluid Volume Evaluation
Amniotic fluid is not measured using AFI (Amniotic Fluid Index) or MVP (Maximum Vertical Pocket) during the first trimester.
Instead, a subjective or qualitative assessment is typically performed. The sonographer ensures adequate amniotic fluid surrounds the fetus for proper NT visualization.
Any significant findings like oligohydramnios (too little fluid) or polyhydramnios (too much fluid) are reported if noted.
2. When is Quantitative AFV Measurement Done?
Quantitative AFV assessment usually begins around 20 weeks during the anomaly scan. It is measured using either:
- AFI – Four-quadrant technique (used in singletons).
- MVP – Maximum Vertical Pocket method (especially in twin pregnancies).
3. Clinical Significance
Although rare, abnormal fluid volume in the first trimester may suggest early rupture of membranes, fetal anomalies, or chromosomal abnormalities.
Suspicious findings require follow-up evaluation and possibly genetic counseling.
3D AFV measurement
1. 3D Volume Acquisition
A 3D transducer captures a complete volume of the amniotic sac using a transabdominal or transvaginal approach depending on fetal position.
2. VOCAL Method (Virtual Organ Computer-aided AnaLysis)
Most common method for calculating 3D fluid volume:
- Freeze the 3D image.
- Manually or semi-automatically trace the amniotic fluid in rotating planes (every 15° or 30°).
- The software reconstructs and displays the calculated AF volume in mL.
Normal AFV Values
Gestational Age (weeks)
11 weeks – ~30–35 mL
12 weeks – ~40–50 mL
13 weeks – ~60–70 mL
Note: While 3D AFV measurements provide promising data, normal ranges are not yet standardized for routine clinical use. Follow-up is essential if abnormalities are suspected.
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