What is the most common cause of asymmetric intrauterine growth restriction (IUGR) in a newborn?

Most Common Cause of Asymmetric Intrauterine Growth Restriction

The most common cause of asymmetric IUGR in a newborn is placental insufficiency, which accounts for 25-30% of all fetal growth restriction cases. 1

Understanding Asymmetric vs. Symmetric IUGR

While the traditional classification of IUGR into symmetric and asymmetric patterns was historically thought to provide valuable prognostic information, recent evidence demonstrates that the HC/AC ratio (which defines asymmetric vs. symmetric IUGR) is not an independent predictor of adverse pregnancy outcomes. 1 Growth and developmental outcomes from birth to age 4 years are similar in both symmetric and asymmetric growth-restricted preterm newborns. 1

Despite this limitation in prognostic value, the asymmetric pattern (characterized by "head-sparing" with preserved head circumference relative to abdominal circumference) remains clinically relevant for understanding the underlying etiology.

Primary Etiologic Categories

Placental insufficiency due to suboptimal perfusion of the maternal-placental circulation is the leading cause, responsible for 25-30% of all FGR cases. 1 This is particularly true for:

• Late-onset FGR (diagnosed ≥32 weeks), which represents 70-80% of all FGR cases and is typically associated with placental underperfusion 1
• Maternal hypertensive disorders and vascular disease 1
• Uteroplacental insufficiency with histologic findings of fibrinoid necrosis, villous fibrosis, avascular terminal villi, and villous infarcts 2

Chromosomal disorders and congenital malformations account for approximately 20% of FGR cases. 1 These are more commonly associated with:

• Early-onset FGR (diagnosed <32 weeks gestation) 1
• Symmetric growth restriction patterns 1

Clinical Context and Timing

The gestational age at diagnosis provides important etiologic clues:

• Earlier in gestation: Chromosomal anomalies, syndromes, and viral infections are more common etiologies 1
• Later in gestation: Placental insufficiency predominates, especially related to hypertension and maternal vascular disease 1

Asymmetric FGR with "head-sparing" is characteristically more common in late-onset cases, where placental insufficiency leads to preferential blood flow redistribution to protect the fetal brain at the expense of abdominal/somatic growth. 1

Important Clinical Pitfall

A critical caveat: approximately 40% of IUGR cases have no identifiable underlying pathology despite thorough evaluation. 3 Among preventable environmental causes, maternal smoking during pregnancy is the single most important modifiable risk factor, responsible for more than one-third of all IUGR newborns. 3

Practical Approach

When evaluating an asymmetric growth-restricted newborn:

• Placental pathology should be examined for lesions of uteroplacental insufficiency, which show the strongest correlation with asymmetric IUGR 2
• Maternal history of hypertensive disorders is a key association with late-onset asymmetric FGR 1
• Detailed fetal structural survey should have been performed prenatally, as approximately 10% of fetuses with FGR have congenital anomalies 1