What is Intrauterine Growth Restriction (IUGR)?

Definition of Intrauterine Growth Restriction (IUGR)

Intrauterine growth restriction (IUGR), now preferably termed fetal growth restriction (FGR), is defined as an estimated fetal weight (EFW) or abdominal circumference (AC) below the 10th percentile for gestational age. 1, 2, 3

Core Diagnostic Criteria

• Standard threshold: FGR is diagnosed when EFW or AC falls below the 10th percentile for gestational age, as established by the American College of Obstetricians and Gynecologists 1, 2, 3

• Severe FGR: Defined as EFW below the 3rd percentile, representing the most critical category with stillbirth rates up to 2.5% 1, 2, 3

Distinguishing IUGR from Related Terms

IUGR versus Small for Gestational Age (SGA): While these terms are often used interchangeably, important distinctions exist 4, 5:

• FGR/IUGR represents a pathological condition where the fetus fails to achieve its genetic growth potential due to placental insufficiency or other pathologic processes 4, 6

• SGA is a statistical definition referring to birthweight below the 10th percentile, which may include constitutionally small but healthy infants 3, 5

• The Society for Maternal-Fetal Medicine recommends using "fetal growth restriction (FGR)" for prenatal diagnosis and reserving "small for gestational age (SGA)" for postnatal classification 3

Pathophysiologic Basis

True IUGR represents a pathological condition where the placenta fails to deliver adequate oxygen and nutrients to the developing fetus, distinguishing it from constitutional smallness 7, 5:

• The condition results from various etiologies including maternal, placental, fetal, and genetic factors 4

• Uteroplacental vascular insufficiency is one of the main causes, leading to chronic oxygen and nutrient deprivation 6

• The growth-restricted fetus displays circulatory adaptations to preserve oxygen and nutrient supply to vital organs (brain, heart, adrenals) 6

Clinical Significance and Risk Stratification

Fetuses below the 10th percentile face doubled stillbirth risk (approximately 1.5% compared to 0.7% in normally growing fetuses), along with increased risks of severe acidosis, low Apgar scores, and NICU admissions 1, 3:

• Below the 5th percentile, stillbirth rates increase to 2.5% 3

• Below the 3rd percentile (severe FGR), stillbirth rates reach up to 2.5% regardless of other findings 3

• Preterm FGR compounds risk with 2- to 5-fold increased perinatal death rates compared to term FGR 1

Confirming Pathological Growth Restriction

Additional parameters beyond size alone help distinguish pathological FGR from constitutional smallness 2, 3:

• Abnormal umbilical artery Doppler: Elevated pulsatility index, resistance index, or absent/reversed end-diastolic velocity confirms placental insufficiency 2, 3

• Reduced growth velocity: AC change <5mm over 14 days or >30% reduction in growth velocity indicates progressive pathology 2, 3

• Oligohydramnios: Suggests chronic placental dysfunction 3

• Abnormal middle cerebral artery Doppler or cerebroplacental ratio: Indicates fetal brain-sparing redistribution 3

Timing Classification

• Early-onset FGR: Diagnosed at <32 weeks gestation, often associated with more severe placental insufficiency and may warrant genetic testing 1, 2

• Late-onset FGR: Diagnosed at ≥32 weeks gestation 2

Long-Term Implications

IUGR is linked with lifelong health consequences beyond the perinatal period 8, 7:

• Impaired intellectual performance in childhood 8

• Adult diseases including hypertension, obesity, cardiovascular disease, metabolic syndrome, diabetes, respiratory diseases, and chronic kidney disease 8, 7

• Neurodevelopmental handicaps and developmental origins of health and disease 5