Being told your baby is measuring small can be frightening. But small for gestational age (SGA) covers a wide spectrum — from perfectly healthy small babies to those with genuine growth restriction. Here's how to tell them apart.
What SGA Means
SGA is defined as an estimated fetal weight (EFW) or birth weight below the 10th percentile for gestational age. This means about 1 in 10 babies will be SGA by definition — most of them are constitutionally small, not unwell.
SGA vs IUGR
| Term | Definition | Concern Level |
|---|---|---|
| Constitutionally small | Below 10th percentile, normal Doppler, appropriate growth rate | Low — healthy small baby |
| SGA (nonspecific) | Below 10th percentile — cause not yet determined | Monitoring required |
| IUGR (growth restriction) | Below 10th percentile AND abnormal Doppler OR static growth | High — placenta may be failing |
Why the Doppler Scan Is Key
The umbilical artery Doppler measures the resistance to blood flow in the cord. In placental insufficiency, resistance rises — in severe cases, the diastolic flow disappears or even reverses. Absent or reversed end-diastolic flow is a sign of significant fetal compromise.
Monitoring Schedule for SGA
- Umbilical artery Doppler every 2 weeks
- Serial biometry scans every 2–3 weeks to check growth velocity
- Biophysical profile or non-stress test for more severe cases
- Hospitalisation and early delivery if Doppler worsens critically
Key insight
The trend matters more than a single measurement. A baby who measures on the 8th percentile consistently, with a normal Doppler, is much less concerning than one who drops from the 30th to the 5th percentile between scans.
Birth Weight and Outcomes
SGA babies (especially IUGR) are at higher risk of stillbirth, neonatal hypoglycaemia, temperature instability, and feeding difficulties. However, with appropriate monitoring and timely delivery, the majority have excellent long-term outcomes.