Small for Gestational Age and IUGR
On this page
Direct answer
Small for gestational age (SGA) is a birth weight below the 10th centile for the baby's gestational age, while intrauterine growth restriction (IUGR) is a pathological failure of the fetus to reach its growth potential. Many SGA babies are constitutionally small and healthy, but IUGR babies face hypoxia and metabolic risk. The key distinction is that SGA is a statistical label and IUGR is a clinical state.
What you must remember
- SGA definition: birth weight below the 10th centile on a standard chart for gestational age, with below minus 2 SD marking a more severe group.
- SGA is not always IUGR: constitutionally small babies grow normally, so SGA alone does not mean the baby is growth restricted.
- Symmetric IUGR: head and body are reduced proportionately after an early insult, such as a chromosomal defect, TORCH infection or maternal alcohol or smoking.
- Asymmetric IUGR: the head is spared while the abdomen is smaller, usually from late placental insufficiency, as in pre-eclampsia or chronic hypertension.
- Doppler: absent or reversed end-diastolic flow in the umbilical artery signals severe placental compromise.
- Neonatal complications: hypoglycaemia, polycythaemia, hypothermia, meconium aspiration and perinatal asphyxia.
- Long-term risk: insulin resistance, type 2 diabetes and hypertension in adult life, especially after rapid catch-up growth.
Symmetric and asymmetric growth restriction
The timing of the insult determines the pattern of growth restriction, and the pattern guides the cause and the prognosis. Examiners usually ask students to recognise the pattern from the head and abdominal measurements. The table below sets out the key differences.
| Feature | Symmetric IUGR | Asymmetric IUGR |
|---|---|---|
| Timing | Early in pregnancy | Late in pregnancy |
| Main causes | Chromosomal, infection, drugs | Placental insufficiency |
| Head and body | Both reduced proportionately | Head spared, abdomen reduced |
| Brain sparing | Absent | Present |
| Common neonatal issue | Structural or genetic features | Hypoxia and metabolic stress |
Symmetric restriction usually reflects an insult that affected the whole fetus from early on, so it often carries an intrinsic cause.
Asymmetric restriction reflects the fetus redirecting blood flow to the brain at the expense of the abdominal organs, which is why it is linked to hypoxia. Clinicians therefore look for the pattern before deciding on the work-up, and the pattern also helps the paediatrician plan the neonatal care needed after birth, including glucose monitoring and temperature control.
Where the exam tries to catch you
SGA and IUGR questions test the definitions, the Doppler findings and the long-term outcomes.
- Trap: calling every small newborn IUGR. Correct: SGA is a centile-based label, and a constitutionally small baby with normal growth and Doppler is not growth restricted in the pathological sense.
- Trap: expecting brain sparing in symmetric restriction. Correct: brain sparing, where the head is preserved relative to the abdomen, is the signature of asymmetric restriction from placental insufficiency.
- Trap: assuming the metabolic risk ends at birth. Correct: rapid catch-up growth after birth adds to the risk of insulin resistance and later obesity, so the risk extends into adult life.
Frequently asked questions
What is the birth weight cut-off for small for gestational age?
Small for gestational age is conventionally defined as a birth weight below the 10th centile for gestational age on a standard growth chart. Some definitions use below minus 2 standard deviations to identify a more severe group. Centile-based criteria are used because normal birth weight varies with gestation and sex.
How does symmetric IUGR differ from asymmetric IUGR?
Symmetric IUGR reduces head and body growth proportionately and starts early in pregnancy, usually from a chromosomal defect, infection or drug exposure. Asymmetric IUGR spares the head and reduces abdominal growth more, usually from late placental insufficiency. The two patterns differ in timing, cause and the head-to-abdomen relationship.
Why are babies with IUGR at risk of adult diabetes?
Babies with IUGR are at risk of adult diabetes because an adverse intrauterine environment may programme insulin resistance and altered metabolism, a concept known as fetal programming. Rapid catch-up growth in childhood can add to this risk. The fetal origins hypothesis explains the link between low birth weight and later metabolic disease.
What does absent end-diastolic flow on umbilical artery Doppler indicate?
Absent or reversed end-diastolic flow in the umbilical artery indicates severe placental resistance and fetal compromise in intrauterine growth restriction. It signals a high risk of fetal hypoxia and stillbirth, so timing of delivery is considered by obstetricians. Doppler is therefore a central tool in monitoring growth-restricted pregnancies.