In fetal growth restriction, how should management proceed when only the head circumference is below the 10th percentile while abdominal circumference and estimated fetal weight are normal?

Management of Isolated Small Head Circumference (<10th Percentile)

When only the head circumference (HC) is below the 10th percentile while abdominal circumference (AC) and estimated fetal weight (EFW) remain normal, this does NOT meet the diagnostic criteria for fetal growth restriction (FGR) and should not be managed as FGR.

Why This Is Not FGR

FGR is specifically defined by AC or EFW <10th percentile—not head measurements alone 1. The 2024 BJOG guideline explicitly states that "fetal size alone is not sufficient to identify FGR, unless AC or EFW is below the 3rd centile" 2. The head circumference is intentionally excluded from the diagnostic criteria because:

• Many constitutionally small fetuses have small heads without pathology 2
• AC is the most sensitive single parameter for detecting placental insufficiency, as the liver (reflected in AC) is preferentially affected in growth restriction
• The standard FGR surveillance protocols (umbilical artery Doppler, delivery timing algorithms) are validated only for fetuses meeting AC/EFW criteria

What To Do Instead

1. Verify Measurements and Dating

• Confirm gestational age accuracy—dating errors are the most common cause of apparent biometric discrepancies
• Repeat ultrasound in 2-3 weeks to assess all biometric parameters and look for:
  • Any decline in AC or EFW percentiles (>50 centile drop suggests FGR) 2
  • Evolving asymmetry or changes in growth velocity 3

2. Evaluate for Alternative Etiologies

Since isolated small HC does not indicate placental insufficiency, consider:

• Genetic/chromosomal abnormalities: Particularly if HC <3rd percentile or if other structural anomalies are present

  • Offer detailed anatomic survey and consider genetic counseling 1
  • Chromosomal microarray if unexplained and early-onset (<32 weeks) 1

• Congenital infections: Screen for CMV, toxoplasmosis if other risk factors present 1

• Microcephaly syndromes: Review for associated structural brain abnormalities on detailed neurosonography

3. Do NOT Initiate FGR Protocols

• Do not perform serial umbilical artery Doppler surveillance—this is only indicated when AC or EFW <10th percentile 1, 4
• Do not alter delivery timing based on isolated small HC—delivery recommendations for FGR (37-39 weeks depending on severity) do not apply 1
• Avoid unnecessary interventions that increase morbidity without evidence of benefit

4. Serial Growth Monitoring

• Repeat ultrasound every 3-4 weeks to monitor all parameters, particularly watching for:
  • AC or EFW crossing below 10th percentile (would then meet FGR criteria)
  • Deceleration in growth velocity across multiple parameters 3, 5
  • Development of oligohydramnios

Key Clinical Pitfall

The most common error is misclassifying isolated small HC as FGR and initiating inappropriate surveillance or early delivery. This exposes the fetus to iatrogenic prematurity without evidence of placental insufficiency. The HC/AC ratio, while sometimes elevated in asymmetric growth patterns, has poor predictive accuracy and should not drive clinical decisions when AC and EFW are normal 3.

When to Escalate Concern

Transition to FGR management protocols only if:

• AC or EFW drops below 10th percentile on subsequent scans 2, 1
• Growth velocity shows >50 centile decline in AC or EFW 2
• Oligohydramnios develops
• Other concerning features emerge (structural anomalies, abnormal Doppler if checked for other indications)

In summary: Isolated small HC with normal AC/EFW requires investigation for non-placental causes and serial monitoring, but does not warrant FGR-specific interventions unless diagnostic criteria are subsequently met.