In a pregnant patient with suspected intrauterine growth restriction, how do symmetric (proportionate) and asymmetric (head‑sparing) IUGR differ in timing, etiology, ultrasound and Doppler findings, and clinical management?

Symmetric vs. Asymmetric IUGR: Key Distinctions

Symmetric IUGR occurs predominantly in early gestation (typically <28-32 weeks) due to chromosomal anomalies, congenital syndromes, and viral infections, while asymmetric IUGR with head-sparing develops later (≥32 weeks) from placental insufficiency related to maternal hypertensive disease. 1

Timing and Gestational Age Thresholds

• Symmetric IUGR is more common earlier in gestation, with proposed thresholds at 28,32, or 34 weeks representing different pathologies and etiologies 1
• Asymmetric IUGR with "head-sparing" characteristically appears later in gestation when placental insufficiency predominates 1, 2
• The distinction matters because early-onset disease (<32 weeks) warrants genetic testing including chromosomal microarray analysis, which provides 4-10% incremental diagnostic yield over standard karyotyping 2

Etiology and Pathophysiology

Symmetric IUGR (Proportionate Growth Restriction)

• Chromosomal anomalies, genetic syndromes, and viral infections are the primary causes in early gestation 1
• Chromosomal disorders and congenital malformations together account for approximately 20% of all FGR cases 2
• All fetal parameters (head circumference, abdominal circumference, femur length) are proportionately reduced 3
• The insult typically occurs early in pregnancy, affecting the fundamental growth potential of all fetal structures 3

Asymmetric IUGR (Head-Sparing Growth Restriction)

• Placental insufficiency is the dominant cause, accounting for 25-30% of all FGR cases 2
• Maternal hypertensive disorders and vascular disease are key associations with late-onset asymmetric FGR 2
• The fetus redistributes blood flow preferentially to protect the brain at the expense of abdominal/somatic growth 2
• Abdominal circumference is disproportionately reduced while head circumference and length are relatively preserved 3

Important caveat: Recent evidence challenges the traditional teaching that asymmetric IUGR is confined to the third trimester. A 2011 study found that 58% of severe, early-onset IUGR cases (<33 weeks) due to placental insufficiency demonstrated an asymmetric phenotype with elevated head circumference/abdominal circumference ratio ≥95th percentile—more than 10-fold greater than expected 4. This suggests asymmetric patterns can occur earlier than previously believed when placental disease is severe.

Ultrasound and Doppler Findings

Biometric Assessment

• Symmetric IUGR: All biometric parameters (biparietal diameter, head circumference, abdominal circumference, femur length) fall proportionately below the 10th percentile 1
• Asymmetric IUGR: Head circumference/abdominal circumference ratio ≥95th percentile indicates preferential head-sparing 4
• Estimated fetal weight (EFW) below the 10th percentile defines FGR; below the 3rd percentile defines severe FGR with stillbirth rates up to 2.5% 5, 6

Doppler Velocimetry Patterns

• Umbilical artery Doppler is essential for differentiating pathological growth restriction from constitutional smallness 6, 7
• Elevated pulsatility index, resistance index, or absent/reversed end-diastolic velocity confirms placental insufficiency 5, 6
• Middle cerebral artery Doppler and cerebroplacental ratio demonstrate brain-sparing redistribution in asymmetric IUGR 1, 5
• Ductus venosus Doppler with reversed A-wave flow is abnormal throughout gestation and indicates severe fetal compromise 1
• Uterine artery Doppler combined with umbilical artery Doppler offers better prediction of adverse perinatal outcomes than either measure alone 1

Additional Sonographic Markers

• Oligohydramnios suggests chronic placental dysfunction 5
• Reduced growth velocity (AC change <5mm over 14 days or >30% reduction in growth velocity) indicates progressive pathology 5, 6
• Approximately 10% of fetuses with FGR have congenital anomalies, and 20-60% of fetuses with congenital anomalies are SGA, necessitating detailed structural surveys 1, 2

Clinical Management Approach

Initial Diagnostic Workup

For Early-Onset IUGR (<32 weeks):

• Perform detailed fetal structural survey to identify the 10% with congenital anomalies 1, 2
• Offer chromosomal microarray analysis (CMA) for unexplained isolated IUGR (GRADE 1C recommendation) 2
• When amniocentesis is performed, offer PCR testing for cytomegalovirus 2
• Do not routinely screen for toxoplasmosis, rubella, or herpes without specific risk factors—this wastes resources and delays appropriate management 2

For Late-Onset IUGR (≥32 weeks):

• Focus on placental insufficiency as the predominant etiology 1
• Evaluate for maternal hypertensive disorders and vascular disease 2
• Genetic testing is less commonly indicated unless structural anomalies are present 2

Surveillance Protocol

For EFW 3rd-9th percentile with normal umbilical artery Doppler:

• Umbilical artery Doppler every 1-2 weeks initially 6
• If stable, extend interval to every 2-4 weeks 6
• Biophysical profile (BPP) remains the mainstay of fetal well-being evaluation, assessing fetal breathing, movements, tone, and amniotic fluid volume 1

For severe IUGR (EFW <3rd percentile):

• Weekly umbilical artery Doppler evaluation 6
• Weekly cardiotocography (CTG) 6
• More intensive monitoring is required regardless of Doppler findings due to 2.5% stillbirth risk 5

For abnormal umbilical artery Doppler (absent/reversed end-diastolic velocity):

• Doppler assessment 2-3 times per week for absent end-diastolic velocity 6
• Add middle cerebral artery and venous Doppler (especially ductus venosus) to assess fetal well-being and assist with delivery timing 1
• Consider hospital admission if fetal surveillance more than 3 times per week is necessary 6

Delivery Timing Considerations

• The predominant theme for all FGR pregnancies is individualized timing of surveillance and delivery based on gestational age, Doppler abnormalities, fetal well-being, and presence of structural defects or chromosomal abnormalities 1
• Preterm FGR carries 2- to 5-fold increased perinatal death rates compared to term FGR 5
• Knowledge of specific gestational age thresholds (28,32, or 34 weeks) helps guide management as these represent varying etiologies and pathologies 1

Long-Term Implications

• IUGR is linked with lifelong health consequences including impaired intellectual performance in childhood and adult diseases such as hypertension, obesity, and cardiovascular disease 5
• Acute neonatal consequences include perinatal asphyxia, respiratory distress from meconium aspiration, persistent pulmonary hypertension, abnormalities of glucose regulation, temperature instability, and polycythemia 3
• IUGR preterm infants face increased incidence of chronic lung disease, necrotizing enterocolitis, retinopathy of prematurity, and postnatal growth failure 3