Differential Diagnoses for IUGR (Fetal Growth Restriction)
The most common cause of fetal growth restriction is suboptimal placental perfusion (25-30% of cases), followed by chromosomal disorders and congenital malformations (approximately 20% of cases). 1
Primary Etiologic Categories
1. Placental Causes (Most Common - 25-30%)
- Uteroplacental insufficiency - the leading pathological mechanism
- Abnormal placental implantation
- Placental infarction or abruption
- Chronic placental inflammation
- Maternal vascular malperfusion
2. Fetal Genetic/Chromosomal Abnormalities (20%)
For unexplained FGR diagnosed before 32 weeks, chromosomal microarray analysis should be offered 1, as this represents a substantial proportion of early-onset cases. Key considerations include:
- Aneuploidy (trisomy 13,18,21, Turner syndrome)
- Genetic syndromes
- Chromosomal microdeletions/duplications
3. Congenital Malformations (Part of 20%)
- Structural cardiac defects
- Renal anomalies
- Central nervous system malformations
- Gastrointestinal abnormalities
- Multiple congenital anomalies
4. Congenital Infections
Screening for TORCH infections (toxoplasmosis, rubella, herpes) is NOT recommended in the absence of other risk factors 1. However, consider:
- Cytomegalovirus - PCR testing recommended for unexplained FGR when amniocentesis is performed 1
- Toxoplasmosis (only with specific risk factors)
- Rubella (only with specific risk factors)
- Syphilis
- Zika virus (in endemic areas)
5. Maternal Medical Conditions
- Chronic hypertension
- Preeclampsia/eclampsia
- Chronic renal disease
- Autoimmune disorders (antiphospholipid syndrome, systemic lupus erythematosus)
- Diabetes with vascular complications
- Cyanotic heart disease
- Chronic hypoxemia (severe pulmonary disease)
- Severe anemia
- Thrombophilias
6. Maternal Behavioral/Environmental Factors
- Cigarette smoking - responsible for over one-third of preventable IUGR cases 2
- Substance abuse (cocaine, heroin, alcohol)
- Malnutrition/severe caloric restriction
- High altitude residence
7. Multiple Gestation
- Twin-to-twin transfusion syndrome
- Selective growth restriction in multiples
- Unequal placental sharing
8. Idiopathic (40% of cases)
Despite thorough evaluation, no underlying pathology can be identified in at least 40% of growth-restricted infants 2. This represents a significant diagnostic challenge and emphasizes the importance of distinguishing true pathological FGR from constitutional smallness.
Clinical Approach to Differentiating Causes
The key is distinguishing pathological FGR from constitutionally small (but healthy) fetuses:
- Normal Doppler studies + growth parallel to percentiles + no anomalies = IUGR unlikely 3
- Abnormal umbilical artery Doppler (especially absent/reversed end-diastolic flow) = placental insufficiency 4
- Early-onset FGR (<32 weeks) = higher likelihood of chromosomal/structural abnormalities 1
- Late-onset FGR (≥32 weeks) = more commonly placental dysfunction 4
Important Diagnostic Pitfall
Do not assume normal umbilical artery Doppler after 32 weeks rules out FGR or fetal compromise 4. Late-onset FGR can occur with normal umbilical artery Doppler but still represent significant placental dysfunction requiring close monitoring.
The genetic contribution accounts for one-third of birth weight variation, while environmental factors account for two-thirds 2, highlighting that many cases result from modifiable or treatable maternal conditions rather than inevitable fetal pathology.