What is Intrauterine Growth Restriction (IUGR)?
Intrauterine growth restriction (IUGR), now preferably termed fetal growth restriction (FGR), is defined as a fetus with an estimated fetal weight (EFW) or abdominal circumference (AC) below the 10th percentile for gestational age, representing a rate of fetal growth that is less than normal for that specific infant's growth potential. 1, 2, 3
Core Definition and Terminology
- IUGR/FGR is diagnosed when the 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 IUGR is defined as EFW below the 3rd percentile, which carries substantially higher risk 1, 2, 3
- The American College of Obstetricians and Gynecologists now recommends using "fetal growth restriction (FGR)" for prenatal diagnosis rather than "intrauterine growth restriction (IUGR)", reserving "small for gestational age (SGA)" for newborns with birthweight below the 10th percentile 3
Critical Distinction: IUGR vs SGA
IUGR and SGA are not interchangeable terms, despite common misuse in clinical practice. 4, 5, 6
- IUGR represents pathological growth restriction where the fetus fails to achieve its genetic growth potential due to placental insufficiency or other pathological processes 4, 5
- SGA simply describes a statistical finding of size below the 10th percentile, which may include constitutionally small but healthy fetuses 3, 6
- IUGR can be confirmed by additional abnormalities including pathological umbilical artery Doppler (elevated resistance, absent or reversed end-diastolic flow), oligohydramnios, reduced growth velocity (AC change <5mm over 14 days or >30% reduction), or abnormal middle cerebral artery/cerebroplacental ratio 2, 3
Clinical Significance and Risks
IUGR is characterized by the American College of Obstetricians and Gynecologists as "one of the most common and complex problems in modern obstetrics." 7
- Fetuses below the 10th percentile have a 1.5% stillbirth rate, which is twice that of normally growing fetuses 3
- Below the 5th percentile, stillbirth rates increase to 2.5% 3
- IUGR is associated with a 2-fold increased risk of stillbirth overall, with additional elevated risks of perinatal asphyxia, severe acidosis, low Apgar scores, and NICU admissions 1
- Preterm IUGR compounds risk significantly, with 2- to 5-fold increased perinatal death rates compared to term IUGR 1
Acute and Long-Term Complications
IUGR infants face both immediate neonatal problems and long-term health consequences. 5
- Acute neonatal complications include perinatal asphyxia, hypothermia, hypoglycemia, and polycythemia 5
- Long-term sequelae include growth retardation, major and subtle neurodevelopmental handicaps, impaired intellectual performance, and increased risk of adult diseases such as hypertension and obesity 7, 5
- Suboptimal growth at birth is linked with diseases in adulthood, highlighting the developmental origins of health and disease 7
Classification by Timing
- Early-onset IUGR is diagnosed at <32 weeks gestation and may warrant genetic testing, including chromosomal microarray analysis when accompanied by fetal malformations or polyhydramnios 2, 3
- Late-onset IUGR is diagnosed at ≥32 weeks gestation 2
Pathophysiology
IUGR results from various maternal, placental, and fetal factors that compromise fetal growth potential. 4, 8
- Maternal nutritional status, diet, and environmental exposures can affect fetal growth by altering nutrient availability and modulating placental gene expression 8
- Placental insufficiency with obliteration of small muscular arteries in the placental tertiary stem villi leads to increased umbilical artery resistance 7
- Oxidative stress and consecutive inflammatory and metabolic changes represent key molecular mechanisms underlying adverse sequelae 6