What if a fetus has asymmetrically small head and femurs (femoral length)?

Asymmetrically Small Fetal Head and Femurs: Evaluation and Management

A fetus with asymmetrically small head and femurs requires comprehensive evaluation for fetal growth restriction (FGR), chromosomal abnormalities, and skeletal dysplasias, with management focused on determining etiology and optimizing outcomes through appropriate surveillance and delivery timing. 1

Clinical Significance and Differential Diagnosis

Primary Concerns

• Fetal Growth Restriction (FGR)

  • Asymmetric FGR with "head-sparing" is typically seen later in gestation (after 32-34 weeks)
  • Earlier in gestation, symmetric FGR is more common and often associated with chromosomal abnormalities 1
  • Approximately 10% of FGR fetuses have congenital anomalies

• Chromosomal Abnormalities

  • Shortened femurs: Associated with trisomy 21 (positive likelihood ratio 1.5-2.7)
  • Shortened head circumference: May indicate microcephaly or other brain abnormalities
  • Combined findings increase suspicion for genetic disorders 1

• Skeletal Dysplasias

  • Many skeletal dysplasias present with measurements below the 3rd percentile
  • Achondroplasia may not manifest until third trimester 1

Diagnostic Approach

1. Detailed Ultrasound Assessment

• Perform comprehensive fetal biometry including:

  • Biparietal diameter/head circumference
  • Abdominal circumference
  • Femur length
  • Humerus length (if not already measured) 1

• Calculate ratios to determine asymmetry:

  • For shortened femur: ratio of observed to expected femoral length <0.92
  • For shortened humerus: ratio of observed to expected humeral length <0.90 1

2. Aneuploidy Risk Assessment

• For isolated shortened femurs/head:
  • Offer cell-free DNA (cfDNA) screening or quad screen if cfDNA unavailable
  • Consider racial/ethnic variation in femur length (shorter in Asian populations, longer in Black populations) 1

3. Doppler Studies

• Umbilical artery Doppler velocimetry

  • Essential for evaluating placental function
  • Combined with uterine artery Doppler provides better prediction of adverse outcomes 1

• Middle cerebral artery Doppler

  • Evaluates fetal brain-sparing effect
  • Cerebroplacental ratio helpful in identifying fetuses at risk 1

4. Additional Imaging

• Consider fetal MRI if CNS abnormalities are suspected 1
• Detailed assessment of all appendicular bones compared with nomograms 1

Management Algorithm

For Suspected FGR

1. Determine timing of onset

   • Early onset (<32 weeks): Higher risk of chromosomal/structural abnormalities
   • Late onset (≥32 weeks): More likely due to placental insufficiency 1

2. Surveillance Protocol

   • Biophysical profile (BPP) at least weekly
   • Doppler studies (umbilical artery, middle cerebral artery)
   • Serial growth scans every 2-3 weeks 1

3. Delivery Planning

   • Individualize based on:
     • Gestational age
     • Severity of growth restriction
     • Doppler abnormalities
     • Biophysical profile results 1

For Suspected Chromosomal Abnormality

1. If no previous aneuploidy screening:

   • Offer cfDNA or quad screen
   • Consider amniocentesis if multiple markers present 1

2. If previous negative screening:

   • With isolated findings: No further aneuploidy evaluation needed
   • With multiple markers: Consider diagnostic testing 1

For Suspected Skeletal Dysplasia

• Thorough evaluation of all long bones
• Third-trimester follow-up ultrasound for reassessment 1

Follow-up Recommendations

• Third-trimester ultrasound for all fetuses with shortened femurs/head to:

  • Reassess growth
  • Evaluate for progression of findings
  • Plan for delivery 1

• Doppler surveillance for FGR cases:

  • Frequency depends on severity and gestational age
  • Abnormal Doppler findings may necessitate delivery 1

Important Considerations

• Racial/ethnic variations in femur length should be considered in the differential diagnosis 1

• Timing of FGR onset has significant implications for etiology and management:

  • Early-onset: Higher association with genetic/structural abnormalities
  • Late-onset: More commonly due to placental insufficiency 1

• Asymmetric vs. symmetric growth restriction provides clues to etiology:

  • Asymmetric (head-sparing): Typically later onset, placental issues
  • Symmetric: Often earlier onset, genetic or infectious causes 1