What estimated fetal weight percentile is required to diagnose fetal growth restriction?

Diagnostic Threshold for Fetal Growth Restriction

Fetal growth restriction (FGR) is diagnosed when either the estimated fetal weight (EFW) OR abdominal circumference (AC) falls below the 10th percentile for gestational age. 1, 2

Primary Diagnostic Criteria

The Society for Maternal-Fetal Medicine establishes a clear diagnostic framework:

• EFW <10th percentile is the traditional criterion for FGR diagnosis 1
• AC <10th percentile is equally valid as a standalone diagnostic criterion, even when EFW is normal 1, 3
• Either parameter below the 10th percentile establishes the diagnosis—you do not need both to be abnormal 2, 3

The expanded definition that includes AC independently improves detection of clinically significant FGR. Studies demonstrate that using AC <10th percentile as an independent criterion identifies an additional 29% of cases with abnormal umbilical artery Doppler findings that would be missed using EFW alone 4. This combined approach (EFW or AC <10th percentile) has superior diagnostic accuracy for predicting small-for-gestational-age neonates compared to using either parameter alone (AUC 0.74 vs. 0.69-0.73) 3.

Severity Stratification

Once FGR is diagnosed, severity determines management intensity:

• Severe FGR: EFW <3rd percentile, regardless of Doppler findings 1, 2

  • Associated with stillbirth rates up to 2.5% 1, 2
  • Requires delivery at 37 weeks 5

• Moderate FGR: EFW 3rd-10th percentile with normal umbilical artery Doppler 1, 2

  • Stillbirth rate approximately 1.5% (twice the rate of normal growth) 1, 2
  • Delivery recommended at 38-39 weeks 1, 5

Essential Prerequisites for Diagnosis

Accurate pregnancy dating is mandatory before diagnosing FGR 1, 2. Dating should be established using first-trimester crown-rump length to either confirm menstrual dates or assign new dates 2. Without proper dating, percentile calculations are meaningless and can lead to misdiagnosis.

Critical Clinical Distinctions

Abandon the term "intrauterine growth restriction (IUGR)" in favor of FGR for prenatal diagnosis 1, 2. Reserve "small for gestational age (SGA)" exclusively for newborns with birthweight <10th percentile 1, 2. These terms are not interchangeable—FGR describes a prenatal diagnosis based on ultrasound parameters, while SGA describes a postnatal finding based on actual birthweight 6.

Common Diagnostic Pitfalls

• Do not dismiss fetuses with AC <10th percentile but normal EFW as having adequate growth 3, 4. These fetuses have a 15% rate of abnormal umbilical artery Doppler findings, identical to those diagnosed by EFW criteria alone 4.

• Do not confuse constitutionally small but healthy fetuses with pathologic FGR 2, 6. The key differentiator is umbilical artery Doppler: abnormal Doppler (elevated resistance, absent or reversed end-diastolic flow) confirms placental insufficiency and pathologic growth restriction 2, 6.

• Fetuses at the 18th percentile do not meet criteria for FGR and require only routine prenatal care without intensified surveillance 7. However, if subsequent ultrasounds show crossing of two percentile lines or fall below the 10th percentile, reevaluation with Doppler surveillance is warranted 7.