What is symmetric Intrauterine Growth Restriction (IUGR) and what causes it?

Symmetric IUGR: Definition and Causes

Symmetric intrauterine growth restriction (IUGR) is characterized by proportionate reduction in fetal weight, length, and head circumference, typically resulting from insults occurring early in pregnancy that affect overall cell division and organ development. 1

Definition and Pattern

Symmetric IUGR presents with:

• Proportionate reduction across all fetal measurements (weight, length, and head circumference all below the 10th percentile) 1
• Head circumference is NOT spared, distinguishing it from asymmetric IUGR where the head is relatively preserved 1
• This pattern indicates a global growth impairment affecting the entire fetus rather than selective redistribution of resources 1

Timing of Onset

• Symmetric IUGR typically originates early in pregnancy (first or early second trimester), when the insult affects fundamental cellular proliferation and organ formation 1
• The early timing means the growth restriction affects all developing structures equally, before the fetus develops compensatory mechanisms like brain-sparing 1

Primary Causes

Chromosomal and Genetic Abnormalities

• Chromosomal disorders and congenital malformations account for approximately 20% of all FGR cases 2
• These genetic abnormalities fundamentally impair cellular growth potential from conception 2

Congenital Infections

• Intrauterine infections (TORCH infections: Toxoplasmosis, Other, Rubella, Cytomegalovirus, Herpes) cause symmetric growth restriction by directly damaging fetal cells during critical developmental periods 3
• These infections interfere with normal cell division and organ development 3

Genetic Syndromes

• Various genetic syndromes that affect overall growth potential produce symmetric patterns 3
• These represent intrinsic fetal factors that limit growth capacity regardless of placental function 3

Severe Early Placental Insufficiency

• When placental dysfunction occurs very early in pregnancy, it can produce symmetric rather than asymmetric patterns 2
• Suboptimal perfusion of the maternal-placental circulation accounts for 25-30% of all FGR cases 2

Clinical Significance

The symmetric pattern carries important prognostic implications:

• Higher likelihood of chromosomal abnormalities or genetic syndromes requiring genetic evaluation 4
• Approximately 10-20% of fetuses with FGR have congenital anomalies or chromosomal disorders 4
• Generally poorer long-term neurodevelopmental outcomes compared to asymmetric IUGR, as the brain growth is also compromised 1
• Lifelong health consequences including impaired intellectual performance in childhood and adult diseases such as hypertension, obesity, and cardiovascular disease 2, 5

Diagnostic Approach

When symmetric IUGR is suspected:

• Perform detailed fetal structural survey to identify congenital malformations 4
• Consider chromosomal microarray analysis (CMA), especially for early-onset FGR (<32 weeks) accompanied by fetal malformations 4
• Evaluate for congenital infections through maternal serologies 3
• Use Doppler studies to differentiate pathological FGR from constitutionally small fetuses 2, 3

Important Caveat

The term "intrauterine growth restriction (IUGR)" should be abandoned in favor of "fetal growth restriction (FGR)" according to current guidelines 2, though the symmetric versus asymmetric classification remains clinically relevant for understanding etiology and prognosis.