Diagnostic Criteria for IUGR Using Abdominal Circumference
Intrauterine growth restriction (IUGR) is diagnosed when the abdominal circumference (AC) falls below the 10th percentile for gestational age, with severe IUGR defined as AC below the 3rd percentile. 1, 2
Primary Diagnostic Criteria
- IUGR is defined as sonographic estimated fetal weight (EFW) below the 10th percentile for gestational age and/or abdominal circumference (AC) below the 10th percentile 1, 3
- Severe IUGR is defined as measurements below the 3rd percentile, which is associated with significantly increased risk of adverse perinatal outcomes 1, 3
- An EFW below the 3rd percentile has been found to represent a more severe form of FGR with a 3-fold increased risk of stillbirth compared to the 3rd-5th percentile group 3, 4
Confirming Pathological Growth Restriction
- Abnormal umbilical artery Doppler (pulsatility index, resistance index, or systolic-to-diastolic ratio >95th percentile) helps differentiate true pathological IUGR from constitutionally small fetuses 1, 3
- Absent or reversed end-diastolic velocity in the umbilical artery indicates more severe IUGR with higher risk of adverse outcomes 1, 3
- Reduced growth velocity (change in AC of <5 mm over 14 days or AC/EFW crossing centiles with >30% reduction) confirms pathological growth restriction 1
Classification Based on Timing
- Early-onset IUGR is diagnosed at <32 weeks gestation and is typically more severe, with more significant placental dysfunction 3, 1
- Late-onset IUGR is diagnosed at ≥32 weeks gestation, representing approximately 70%-80% of FGR cases and is typically milder 3, 1
Diagnostic Accuracy
- AC measurement alone has limited sensitivity for detecting IUGR, with a likelihood ratio of only 1.2 (95% CI 1.0-1.4) 5
- The combination of both AC and EFW below the 10th percentile provides better diagnostic accuracy with a likelihood ratio of 2.8 (95% CI 1.6-4.9) for detecting small for gestational age infants 5
- Estimated fetal weight below the 10th percentile has been shown to have a sensitivity of 87% and specificity of 87% for identifying IUGR infants 6
Management After Diagnosis
- Once IUGR is diagnosed using AC measurements, serial umbilical artery Doppler assessment should be performed to assess for deterioration 1, 2
- Initial assessment should occur every 1-2 weeks, with less frequent intervals (every 2-4 weeks) if umbilical artery Doppler remains normal 1
- For severe IUGR (AC <3rd percentile), weekly umbilical artery Doppler evaluation and weekly cardiotocography should be performed 1
Pitfalls and Caveats
- Accurate pregnancy dating is essential for correctly interpreting AC percentiles, with first-trimester crown-rump length being the most reliable method 2
- AC measurement alone may miss cases of asymmetric IUGR where head circumference is preserved ("head-sparing") 3
- The traditional classification of symmetric versus asymmetric IUGR based on HC/AC ratio has limited prognostic value and is not recommended as an independent predictor of adverse outcomes 3
- Fetal diagnostic testing, including chromosomal microarray analysis, should be considered when unexplained isolated IUGR is diagnosed at <32 weeks of gestation 2