How to measure abdominal circumference (AC) in a fetus?

How to Measure Fetal Abdominal Circumference (AC)

Fetal abdominal circumference should be measured using transabdominal ultrasound at a standardized axial plane perpendicular to the fetal spine, at the level where the stomach bubble and umbilical vein are visualized, using either direct tracing with a digitizer or calculation from anteroposterior and transverse diameters. 1

Optimal Imaging Plane and Anatomical Landmarks

The correct measurement plane is critical for accuracy:

• Position the transducer to obtain an axial (cross-sectional) view perpendicular to the fetal longitudinal axis at the level of the fetal abdomen 2, 3
• Identify three key anatomical structures in the same plane: the stomach bubble, the umbilical vein (at the portal sinus), and the spine 3, 1
• The section should be as circular as possible, avoiding oblique cuts that create an oval shape 4
• Ensure the ribs are symmetrical on both sides to confirm proper perpendicular alignment 2

Measurement Technique

Two validated methods exist, with direct measurement being more accurate:

Direct Measurement (Preferred)

• Use electronic calipers or a digitizer to trace the outer perimeter of the fetal abdomen at the skin line 5, 2
• This method provides measurements that are significantly larger and more accurate than calculated measurements (P < 0.0001) 5
• Mean percentage error is approximately 5.0% when compared to postnatal measurements 4

Calculated Measurement (Alternative)

• Measure the anteroposterior (AP) diameter and transverse diameter of the abdomen at the same standardized plane 5
• Calculate AC using the formula: AC = π × (AP + transverse diameter)/2 5
• Be aware this method systematically underestimates AC compared to direct measurement throughout gestation (18-41 weeks) 5

Common Pitfalls to Avoid

• Do not measure at oblique angles - this creates falsely elevated measurements due to an oval rather than circular cross-section 4
• Do not include fetal limbs in the measurement plane - ensure arms and legs are not crossing the abdomen 2
• Avoid measuring when the fetus is in extreme flexion or extension - this distorts the abdominal shape 2
• Do not use calculated AC values interchangeably with direct measurement normative data - they are not equivalent and require different reference ranges 5

Clinical Applications and Timing

• AC is routinely measured as part of the detailed anatomic survey at 18-22 weeks gestation 6, 1
• For growth assessment, repeat measurements should be performed no more frequently than every 2 weeks due to measurement variability 1
• AC is the single best biometric parameter correlated with fetal growth, though it is also the most technically challenging to measure accurately 2
• AC measurements are essential for calculating estimated fetal weight (EFW) and assessing growth restriction 1

Key Diagnostic Thresholds

• AC <10th percentile is a contributory parameter for selective fetal growth restriction in twin pregnancies 1
• The femur length/AC ratio <0.16 is the single best predictor of lethal skeletal dysplasia 1
• Chest circumference/AC ratio <5th percentile suggests pulmonary hypoplasia 1