Asymmetric Intrauterine Growth Restriction: Causes
Asymmetric IUGR is primarily caused by placental insufficiency due to suboptimal uteroplacental perfusion, which accounts for 25-30% of all fetal growth restriction cases and is the predominant etiology in late-onset (third trimester) presentations. 1, 2
Primary Etiology: Placental Insufficiency
Placental insufficiency represents the leading cause of asymmetric IUGR, particularly in late-onset cases (≥32 weeks gestation). 1, 2 This pathophysiology results in:
- Preferential blood flow redistribution to protect the fetal brain at the expense of abdominal and somatic growth, creating the characteristic "head-sparing" pattern seen in asymmetric IUGR 1
- Poor placental function and hypoxemia that manifests as impaired fetal growth 3
Maternal Vascular and Hypertensive Disorders
Maternal hypertensive disorders are a key association with late-onset asymmetric FGR, as these conditions directly compromise uteroplacental perfusion. 1 Specific maternal factors include:
- Chronic maternal diseases affecting placental perfusion 2
- Pre-eclampsia-related placental changes 4
- Maternal vascular disease 1
Timing-Dependent Etiologic Patterns
The etiology of asymmetric IUGR varies significantly by gestational age at presentation:
Late-Onset (Third Trimester)
- Placental insufficiency predominates, especially related to hypertension and maternal vascular disease 1
- Asymmetric FGR with head-sparing is characteristically more common in these late-onset cases 1
Early-Onset (Before 32 weeks)
- Chromosomal anomalies, syndromes, and viral infections are more common etiologies 1
- Chromosomal disorders and congenital malformations account for approximately 20% of FGR cases overall 1, 2
Pathophysiologic Classification
The distinction between asymmetric and symmetric IUGR reflects different underlying mechanisms:
- Asymmetric IUGR (nutritional IUGR): Results from placental insufficiency occurring later in pregnancy, leading to redistribution of blood flow that spares the brain while compromising abdominal/liver growth 4
- Symmetric IUGR (reduced growth potential): Typically results from early insults affecting overall growth potential 4
Additional Contributing Factors
Beyond placental insufficiency, other etiologies that may present with or contribute to asymmetric patterns include:
- Fetal factors: Genetic diseases, congenital malformations, infections, multiple gestations, and placental/cord abnormalities 5
- Maternal factors: Decreased uteroplacental blood flow, reduced blood volume, decreased oxygen carrying capacity, poor nutrition status, teratogens, and socioeconomic factors 5
Critical Clinical Pitfall
Approximately 10-20% of fetuses with FGR have congenital anomalies or chromosomal disorders, making detailed fetal structural survey essential even when placental insufficiency appears to be the primary cause. 2, 6 This is particularly important in early-onset cases where chromosomal abnormalities are more prevalent. 1