Small for Gestational Age (SGA) and Fetal Growth Restriction (FGR)
RCOG Small-for-Gestational-Age Fetus and a Growth Restricted Fetus, Investigation and Care (Green-top Guideline No. 31). Last reviewed: May 2024.
Definition
Small for gestational age (SGA) and fetal growth restriction (FGR) are closely related but distinct terms.
| Term | Definition |
|---|---|
| Small for gestational age (SGA) | EFW or AC <10th centile with or without growth restriction |
| Fetal growth restriction (FGR) | ANY of the following criteria can be used to identify FGR:
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Most SGA fetuses are not FGR. Many fetuses with growth below the 10th centile are constitutionally small and healthy.
Note on terminology: intrauterine growth restriction (IUGR) is the older terminology for fetal growth restriction (FGR). RCOG have shifted to using FGR as the preferred and more accurate term.
Classification
FGR was previously described as symmetrical or asymmetrical because it was proposed that this might help determine the possible underlying aetiology.
RCOG states that these terms should no longer be used as a description of FGR, as the symmetry of fetal growth varies significantly and is not prognostic of the actual pregnancy outcome.
FGR is now classified based on the timing of onset:
| Early-onset FGR | Late-onset FGR | |
|---|---|---|
| Definition | Onset before 32 weeks | Onset from 32 weeks onwards |
| Underlying mechanism | Severe placental dysfunction (notable hypertensive disorders of pregnancy) | Milder placental dysfunction |
| Investigatios | Hallmark: abnormal umbilical artery Doppler
Abnormal ductus venosus Doppler |
Normal umbilical artery Doppler is common, but does NOT exclude it |
| Complications | Significantly increased risk of perinatal mortality and morbidity (from fetal hypoxia and profound fetal cardiovascular adaptations) | Relatively lower risk of severe adverse events |
Aetiology and Risk Factors
There are 3 main aetiological categories:
| Category | Causes and risk factors |
|---|---|
| Normal / constitutionally small | These fetuses are healthy, but simply genetically destined to be small.
Their size is appropriate for their genetic growth potential, and they do not have any pathological restrictions. *This can only cause SGA, but not FGR. |
| Placenta-dysfunction | Maternal conditions (affect placenta implantation and vasculature):
A history of previous pre-eclampsia, placental abruption, previous FGR, or stillbirth is also associated with placental dysfunction Factors that affect placental transfer of nutrients:
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| Inherent fetal conditions |
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Complications
| Complication | Description |
|---|---|
| Stillbirth and mortality | Primarily associated with FGR |
| Intrapartum complications | Associated with both SGA and FGR
Fetuses at term and near-term are at increased risk of fetal heart rate abnormalities and decelerations during labour
|
| Neonatal ICU admission | Associated with both SGA and FGR |
| Long-term neurodevelopmental outcomes | Primarily associated with FGR
|
Surveillance and Detection
Surveillance is determined by the woman’s risk level (risk assessment is continuous and dynamic).
Key principles:
- Low risk = clinical SFH measurement from 24 weeks onwards (ultrasound is only offered if SFH is abnormal)
- Moderate risk = late ultrasound screening from 32 weeks onwards
- High risk = uterine artery doppler triage with ultrasound screening (frequency and timing depend on the uterine artery doppler findings)
Key ultrasound measurements to obtain:
- Estimated fetal weight – derived from the head circumference, abdominal circumference and femur length
- Abdominal circumference
Routine third-trimester ultrasounds should NOT be offered to low-risk, uncomplicated pregnancies.
Work Up
Following the detection of SGA or FGR, the following workup should be performed:
| Category | Investigation / referral | Indications |
|---|---|---|
| Referrals | Fetal medicine specialist |
|
| Tertiary Level Unit (with highest level NICU) |
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| Maternal investigations | Check blood pressure and for proteinuria |
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| CMV and toxoplasmosis serology |
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| Malaria and Zika virus testing | Consider in:
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| Fetal investigations | Umbilical artery Doppler |
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| Fetal movement check and CTG | ||
| Ductus venosus Doppler |
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| Cerebral Dopplers (MCA / CPR / UCR) | Can be used as an adjunct:
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| Invasive diagnostic testing (amniocentesis / CVS) |
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Management
There are currently no medical interventions to treat or reverse SGA or FGR other than the planned birth of the baby (to reduce the risk of stillbirth)
Do NOT offer LMWH and PDE5 inhibitors (e.g. sildenafil) to treat FGR.
SGA Management
Consider induction of labour at 39 weeks and before 39+6 weeks
- Mechanical methods are preferred over pharmacological methods (see the Induction of Labour article for more information)
- Offer continuous CTG monitoring from the onset of contractions
FGR Management
| Timing of birth | Late FGR (≥32 weeks): delivery at 37-37+6 weeks
Preterm delivery (<37 weeks) is indicated if:
|
| Early FGR (<32 weeks): delivery timing is individualised, managed by a tertiary fetal medicine unit | |
| Mode of birth | Induction of labour is generally safe for late FGR if the umbilical artery Doppler shows +ve end-diastolic flow
Planned Caesarean birth is recommended for severe FGR, including:
|
If preterm birth (<37 weeks) is anticipated, planned, or imminent, offer:
- Antenatal corticosteroids between 24-34+6 weeks (ideally 48 hours before an anticipated birth) – to aid fetal lung maturation and reduce risk of NRDS
- Antenatal magnesium sulfate between 24-29+6 weeks (and consider up to 33+6 weeks) – to provide fetal neuroprotection