What is Intrauterine Growth Restriction (IUGR)?
Intrauterine growth restriction (IUGR), now preferably termed fetal growth restriction (FGR), is a condition where a fetus fails to achieve its genetic growth potential, defined as an estimated fetal weight (EFW) or abdominal circumference (AC) below the 10th percentile for gestational age. 1, 2, 3
Definition and Diagnostic Criteria
IUGR/FGR is diagnosed when the EFW or AC falls below the 10th percentile for gestational age, distinguishing it from a constitutionally small but healthy fetus. 1, 2
Severe FGR is defined as EFW below the 3rd percentile, which carries significantly higher risks regardless of other findings. 1, 2, 3
The American College of Obstetricians and Gynecologists characterizes IUGR as "one of the most common and complex problems in modern obstetrics" due to variable definitions, poor detection rates, limited treatment options, and associated morbidities. 4
Distinction from Small for Gestational Age (SGA)
While IUGR and SGA are often used interchangeably, there are important differences:
IUGR/FGR refers to a pathological condition where the fetus fails to grow according to its genetic potential, typically due to placental insufficiency or other pathological processes. 5, 6
SGA is a postnatal term describing newborns with birthweight below the 10th percentile, who may be constitutionally small and healthy rather than pathologically growth-restricted. 3, 6
The Society for Maternal-Fetal Medicine recommends using "fetal growth restriction (FGR)" for prenatal diagnosis and reserving "SGA" for postnatal classification. 3
Underlying Pathophysiology
True IUGR represents a pathological condition where the placenta fails to deliver adequate oxygen and nutrients to the developing fetus, most commonly due to uteroplacental vascular insufficiency. 6, 7, 8
The growth-restricted fetus develops circulatory adaptations to preserve oxygen and nutrient supply to vital organs (brain, heart, adrenals), known as "brain-sparing" redistribution. 7
IUGR results from various etiologies including maternal factors (hypertension, malnutrition), placental factors (insufficiency, infarction), fetal factors (infections, chromosomal abnormalities), and genetic factors. 5
Clinical Significance and Risks
Fetuses with IUGR face substantially elevated risks compared to appropriately grown fetuses:
Stillbirth risk is doubled (approximately 1.5%) for fetuses below the 10th percentile, increasing to 2.5% for those below the 5th percentile. 1, 3
Perinatal risks include perinatal asphyxia, severe acidosis, low Apgar scores, and increased NICU admissions. 1, 6
Preterm IUGR compounds risk with 2- to 5-fold increased perinatal death rates compared to term IUGR. 1
Acute Neonatal Complications
IUGR infants face multiple immediate postnatal challenges:
- Acute complications include perinatal asphyxia, hypothermia, hypoglycemia, and polycythemia, requiring vigilant neonatal monitoring. 3, 6
Long-Term Health Implications
Suboptimal fetal growth leads to lasting physiological alterations with consequences extending into adulthood:
Long-term risks include impaired intellectual performance, neurodevelopmental handicaps, growth retardation, and increased susceptibility to adult chronic diseases. 4, 3, 6
Adult health consequences include hypertension, obesity, cardiovascular disease, metabolic syndrome, diabetes, respiratory diseases with impaired lung function, and chronic kidney disease. 4, 8
These prenatal circulatory adaptations "program" the fetus for lifelong cardiovascular morbidities through altered cardiovascular development. 7, 8
Classification Systems
IUGR is classified by timing and pattern:
Early-onset IUGR occurs before 32 weeks gestation, often associated with more severe placental insufficiency and may warrant genetic testing. 2, 3
Late-onset IUGR occurs at or after 32 weeks gestation. 2
Symmetric IUGR involves proportional reduction in all fetal measurements, often suggesting early insult or genetic/chromosomal causes. 9
Asymmetric IUGR shows head-sparing with disproportionately reduced abdominal measurements, typically indicating placental insufficiency. 9
Confirming Pathological Growth Restriction
Beyond size alone, additional parameters help distinguish true pathological restriction:
Abnormal umbilical artery Doppler (elevated pulsatility index, resistance index, or absent/reversed end-diastolic flow) confirms placental insufficiency. 2, 3
Reduced growth velocity (AC change <5mm over 14 days or >30% reduction crossing centiles) indicates progressive pathology. 2, 3
Abnormal middle cerebral artery Doppler or cerebroplacental ratio indicates brain-sparing redistribution. 3
Oligohydramnios suggests chronic placental dysfunction. 3