What is the definition and cause of Intrauterine Growth Restriction (IUGR) in a pregnant individual in their 3rd trimester?

Definition of IUGR in the Third Trimester

Fetal growth restriction (FGR)—the preferred term over IUGR—is defined as an estimated fetal weight (EFW) or abdominal circumference (AC) below the 10th percentile for gestational age, with severe FGR defined as EFW below the 3rd percentile. 1, 2

Diagnostic Criteria

• Standard FGR: EFW or AC <10th percentile for gestational age 1, 2, 3
• Severe FGR: EFW <3rd percentile, representing the highest-risk category with stillbirth rates up to 2.5% 1, 2
• The term "intrauterine growth restriction (IUGR)" should be abandoned in favor of "fetal growth restriction (FGR)" according to current guidelines 1

Distinguishing Pathological from Constitutional Small Fetuses

Not all small fetuses are pathologically growth-restricted. Abnormal findings that confirm true placental insufficiency include: 2, 3

• Elevated umbilical artery Doppler pulsatility index, resistance index, or absent/reversed end-diastolic velocity
• Reduced growth velocity (AC change <5mm over 14 days or >30% reduction in growth velocity)
• Oligohydramnios indicating chronic placental dysfunction
• Abnormal middle cerebral artery Doppler or cerebroplacental ratio showing brain-sparing redistribution

Causes of FGR in the Third Trimester

Placental Insufficiency (Most Common)

Suboptimal perfusion of the maternal-placental circulation accounts for 25-30% of all FGR cases and is the predominant cause in the third trimester. 1

• Maternal hypertensive disorders 1
• Maternal vascular disease 1
• Placental abnormalities including dysregulated villous vasculogenesis and fibrin deposition 4
• Abnormal spiral arteriole remodeling 4

Fetal Factors

• Chromosomal disorders and congenital malformations account for approximately 20% of FGR cases 1
• Genetic factors contribute to one-third of birth weight variations 5

Maternal Factors

• Maternal smoking is the single most important preventable cause, responsible for more than one-third of all FGR cases 5
• Chronic maternal diseases affecting placental perfusion 1

Idiopathic

• In at least 40% of FGR cases, no underlying pathology can be identified 5

Clinical Significance in Third Trimester

Fetuses with EFW below the 10th percentile face doubled stillbirth risk (approximately 1.5% versus 0.7% in normally growing fetuses), along with increased risks of severe acidosis, low Apgar scores, and NICU admissions. 2

• Below the 5th percentile: stillbirth rates increase to 2.5% 2
• Below the 3rd percentile (severe FGR): stillbirth rates reach up to 2.5% regardless of other findings 2
• Late-onset FGR (diagnosed ≥32 weeks) typically has better outcomes than early-onset FGR but still requires intensive surveillance 2, 3

Long-Term Implications Beyond the Perinatal Period

FGR is associated with lifelong health consequences including impaired intellectual performance in childhood and adult diseases such as hypertension, obesity, and cardiovascular disease through metabolic programming. 2, 6

Common Pitfalls

• Confusing constitutionally small fetuses with pathological FGR: Always use Doppler studies and growth velocity assessment to differentiate 1, 2
• Inaccurate gestational age dating: FGR diagnosis requires accurate gestational age; if uncertain, repeat evaluation to assess growth velocity is mandatory 1
• Using inappropriate growth curves: Use population-specific growth curves that approximate the population being studied 1
• Inadequate surveillance intervals: Assessment of fetal growth should occur at intervals of no less than every 2 weeks, preferably every 3-4 weeks given inherent measurement error 1