At 38 weeks gestation, the fetal abdominal circumference is at the 2.3 percentile and the estimated fetal weight is at the 20th percentile; what is the likely diagnosis and recommended management?

Asymmetric Fetal Growth Restriction with Severe Abdominal Circumference Lag

This fetus has severe fetal growth restriction (FGR) with marked asymmetry, requiring immediate umbilical artery Doppler assessment, detailed anatomical survey, and consideration of genetic testing given the severity of abdominal circumference restriction at the 2.3 percentile. 1, 2

Likely Diagnosis

• Severe asymmetric FGR is the primary diagnosis, defined by abdominal circumference below the 3rd percentile (2.3 percentile qualifies) while the estimated fetal weight remains at the 20th percentile 1, 3

• The marked discrepancy between abdominal circumference and estimated fetal weight suggests placental insufficiency as the underlying etiology, where the fetus redistributes blood flow to preserve brain growth at the expense of liver glycogen stores and abdominal growth 4, 2

• This pattern carries a 3-fold to 7-fold increased risk of stillbirth compared to fetuses with growth parameters between the 5th-10th percentile 2

Immediate Diagnostic Workup

Ultrasound Assessment

• Perform umbilical artery Doppler immediately to stratify risk and determine surveillance intensity, as this is the single most important prognostic tool for management decisions 1, 3

• Obtain a detailed anatomical ultrasound examination to evaluate for structural abnormalities, as up to 20% of early-onset FGR cases (though this is late-onset at 38 weeks, the severity warrants this evaluation) are associated with fetal or chromosomal abnormalities 1, 2

• Assess amniotic fluid volume as oligohydramnios combined with severe FGR significantly worsens prognosis and indicates severe placental dysfunction 4, 3

• Evaluate placental location, appearance, and structural abnormalities to help determine the etiology of growth restriction 2

Genetic and Infectious Considerations

• Offer chromosomal microarray analysis if fetal malformations or polyhydramnios are present alongside the severe abdominal circumference restriction, regardless of gestational age 2, 3

• Do not routinely screen for toxoplasmosis, rubella, or herpes without other risk factors, as these tests have low yield 3

• Consider CMV PCR testing only if diagnostic amniocentesis is elected for other indications, as CMV is the most common infectious cause of FGR worth investigating 3

Maternal Evaluation

• Monitor closely for hypertensive disorders of pregnancy, as maternal hypertension is present in 50-70% of early-onset FGR cases and significantly impacts outcomes 2

Management Based on Doppler Findings

The management algorithm depends critically on umbilical artery Doppler results at 38 weeks:

If Normal Umbilical Artery Doppler

• Deliver at 38-39 weeks for FGR with estimated fetal weight or abdominal circumference between 3rd-10th percentile and normal Doppler 1, 3

• Given this fetus is already at 38 weeks with severe abdominal circumference restriction (2.3 percentile, below 3rd percentile), delivery at 38 weeks is recommended 1

If Decreased End-Diastolic Velocity

• Deliver at 37 weeks for decreased diastolic flow or severe FGR (abdominal circumference <3rd percentile) 1, 3

• Since this fetus is already at 38 weeks, proceed with delivery promptly 1

If Absent End-Diastolic Velocity

• Deliver at 33-34 weeks is the standard recommendation, but at 38 weeks gestation, immediate delivery is indicated 1, 3

• Increase Doppler assessment frequency to 2-3 times per week if expectant management is briefly considered, though at 38 weeks this is unlikely 3

If Reversed End-Diastolic Velocity

• Immediate hospitalization and delivery is required at 38 weeks 1, 3

• Administer antenatal corticosteroids if delivery anticipated before 33 6/7 weeks (not applicable at 38 weeks) 3

Mode of Delivery Considerations

• Consider cesarean delivery based on the entire clinical scenario if absent or reversed end-diastolic velocity is present, as these fetuses have limited physiologic reserve and may not tolerate labor well 3

• Continuous intrapartum fetal monitoring is essential given the increased risk of fetal distress, with a 35% cesarean section rate for fetal distress when abdominal circumference is ≤5th percentile 5

Surveillance If Delivery Delayed

If delivery is delayed beyond 38 weeks for any reason (though not recommended):

• Weekly umbilical artery Doppler evaluation for severe FGR with normal or decreased end-diastolic velocity 1, 3

• Weekly cardiotocography (NST/BPP) after fetal viability when Doppler shows normal or decreased flow 3

• Do not rely on biophysical profile or nonstress testing alone without Doppler assessment, as Doppler provides superior prognostic information 3

Critical Pitfalls to Avoid

• Do not delay delivery beyond 38-39 weeks with this degree of abdominal circumference restriction, as the risks of stillbirth increase significantly 1, 2

• Do not use middle cerebral artery, ductus venosus, or uterine artery Doppler for routine clinical decision-making at this gestational age, as randomized trials have not demonstrated benefit for delivery timing decisions 3

• Abdominal circumference <10th percentile is more sensitive than estimated fetal weight alone for detecting small-for-gestational-age infants (64% vs 50.6% sensitivity), and using either criterion improves detection 6

• Do not prescribe sildenafil, low-molecular-weight heparin, or recommend activity restriction, as these interventions are ineffective and potentially harmful 1, 3