Overview
Small for Gestational Age (SGA) & Fetal Growth Restriction (FGR)
1. Topic Overview (Clinical Overview)
Summary
Small for Gestational Age (SGA) refers to a fetus or neonate with an Estimated Fetal Weight (EFW) or birthweight below the 10th centile for gestational age. It is a descriptive term. Crucially, not all SGA babies are "growth restricted" – some are constitutionally small (healthy parents, healthy baby, just genetically small). Fetal Growth Restriction (FGR) is the pathological subset where the fetus has failed to reach its growth potential due to an underlying cause (usually placental insufficiency). Distinguishing the two is the key clinical challenge because FGR carries a significant risk of stillbirth while constitutional SGA does not.
Key Facts
- Definition (SGA): EFW or Birthweight <10th centile.
- Severe SGA: EFW <3rd centile.
- FGR Definition: SGA + Evidence of pathology (Abnormal Doppler, Reduced AFI, Crossing centiles).
- Most Common Cause: Placental Insufficiency (Pre-eclampsia, HTN, Thrombophilia).
- Key Investigation: Umbilical Artery Doppler Pulsatility Index (UA PI).
- Critical Sign: Absent/Reversed End Diastolic Flow (AEDF/REDF) = Fetus is running out of reserve.
- Key Guideline: RCOG Green-top Guideline 31.
Clinical Pearls
"SGA is a size. FGR is a disease." Always ask: Is this baby just genetically small, or is something wrong?
The Doppler Cascade: As placental function fails, Dopplers deteriorate in a predictable order: Umbilical Artery (resistance up) -> Middle Cerebral Artery (vasodilation/"brain sparing") -> Ductus Venosus (A-wave reversal = heart failure) -> CTG abnormalities -> Stillbirth.
Customised Centiles: A baby at the 8th centile for a healthy tall Scandinavian woman is very different from one at the 8th centile for a petite South Asian woman. Customised centile charts (GROW) account for maternal ethnicity, height, weight, and parity.
Why This Matters Clinically
Unrecognised FGR is a leading cause of preventable stillbirth. The "Saving Babies' Lives" care bundle in the UK specifically targets the detection and management of FGR. Early identification allows appropriate surveillance and timely delivery, preventing tragedy.
2. Epidemiology
Incidence & Prevalence
- SGA (BW <10th centile): 10% by definition.
- Severe SGA (<3rd centile): 3%.
- True FGR: ~3-5% of pregnancies.
Drill Down: Symphysis-Fundal Height (SFH) Measurement
The clinical screening tool for SGA.
- Technique: Measure from the top of the symphysis pubis to the top of the uterine fundus with a tape measure.
- Expected: SFH (cm) ≈ Gestational Age (weeks) from 24 weeks onwards.
- Abnormal: SFH <3rd centile or a "flattening" of the growth trajectory.
- Actions: Single measurement <3rd centile -> USS. Two static measurements (no growth) -> USS.
- Accuracy: Sensitivity ~50-60%. It misses a lot. But it's cheap and universal.
Drill Down: Customised Centile Charts (GROW)
Not all 10th centiles are equal.
- Problem: Population-based centiles (e.g., "10th centile for UK") ignore that a baby's optimal growth depends on maternal characteristics.
- Solution: Customised Centiles adjust for:
- Maternal Ethnicity
- Maternal Height
- Maternal Booking Weight
- Parity
- Tool: GROW (Gestation Related Optimal Weight) charts. Integrated into software (e.g., Badgernet).
- Benefit: Improves detection of "pathologically small" babies (who would otherwise be labelled "normal") and avoids over-investigating "constitutionally small" babies.
Risk Factors (RCOG GTG 31)
Women with these factors should receive additional surveillance.
High-Risk (Require Serial Growth Scans + Doppler):
| Risk Factor | Relative Risk |
|---|---|
| Previous SGA Baby (<10th centile) | 2.5 |
| Previous Stillbirth | 2-4 |
| Pre-eclampsia (Current or Previous) | 2-3 |
| Chronic Hypertension | 2 |
| Antiphospholipid Syndrome | 6 |
| Smoking (>0/day) | 2 |
| Cocaine Use | High |
| BMI >5 | 1.5 |
| Age >0 | 1.5 |
Minor Risk (May warrant increased surveillance):
- Maternal medical disease (Diabetes, Renal, Autoimmune).
- Multiple pregnancy.
- Advanced maternal age (>35).
3. Pathophysiology
Distinction: Constitutional SGA vs. FGR
| Feature | Constitutional SGA | FGR (Placental) |
|---|---|---|
| Definition | Genetically small, healthy baby. | Failed to reach growth potential. |
| Parents | Small parents. | Variable. |
| Dopplers (UA) | NORMAL | Abnormal (High PI, AEDF, REDF). |
| AFI | Normal | Often Oligohydramnios. |
| Growth Velocity | Tracks along centile. | Crosses centiles downwards. |
| Outcome | Good. | Risk of Stillbirth, Hypoxia, NEC. |
The Placental Cascade (FGR Pathophysiology)
The fetus fights to survive placental failure.
- Placental Insufficiency: Poor trophoblast invasion -> High resistance in placental bed.
- Umbilical Artery: Detects increased resistance (High Pulsatility Index). Diastolic flow reduces.
- Fetal Response (Brain Sparing): To protect the brain, the fetus dilates cerebral vessels (MCA). Blood flow is diverted to brain, heart, adrenals. Gut and kidneys are "sacrificed" -> Oligohydramnios.
- Ductus Venosus Changes: The DV shunts oxygenated blood from the UV to the heart. If DV flow reverses during atrial contraction (A-wave reversal), this signals cardiac failure.
- CTG Changes: Reduced variability, late decelerations, sinusoidal pattern.
- Stillbirth: If not delivered in time.
Drill Down: Types of FGR
| Type | Onset | Cause | Dopplers | Prognosis |
|---|---|---|---|---|
| Early-Onset FGR (<32 weeks) | Early | Severe placental dysfunction (Pre-eclampsia, Infection, Chromosomal). | Abnormal early. | Worse – need iatrogenic preterm delivery. |
| Late-Onset FGR (>2 weeks) | Late | Mild placental dysfunction. Senescent placenta. | May be normal initially. | Better. May present as stillbirth if missed. |
4. Clinical Presentation
Antenatal Suspicion
Ultrasound Findings
| Finding | Interpretation |
|---|---|
| EFW <10th Centile | Definition of SGA. |
| EFW <3rd Centile | Severe SGA. High risk of FGR. |
| Abdominal Circumference (AC) <10th | Often the first parameter to lag (liver glycogen depleted). |
| Head:Abdomen Ratio Asymmetry | Classic "head-sparing" FGR. Head normal, AC small. |
| Oligohydramnios (AFI <5) | Reduced renal perfusion. Sign of hypoxia. |
| Crossing Centiles | Growth velocity slowing. Very concerning. |
Fundal Height (SFH)
Measures small for dates (>3cm below expected).
Reduced Fetal Movements
Fetus conserving energy.
Risk Factor Screening
Identified at booking.
5. Doppler Assessment (Key Investigation)
Umbilical Artery (UA) Doppler
Measures placental resistance.
| Finding | Interpretation | Action |
|---|---|---|
| Normal PI | Placental resistance normal. Constitutional SGA likely. | Serial growth scans. |
| Raised PI (>5th centile) | Placental resistance increased. FGR confirmed. | Increase surveillance. |
| Absent End Diastolic Flow (AEDF) | No forward flow in diastole. Severe FGR. | Admit. Steroids. Daily monitoring. Delivery plan. |
| Reversed End Diastolic Flow (REDF) | Flow goes BACKWARDS in diastole. Critical hypoxia. | Imminent delivery (often within 48-72 hours). |
Middle Cerebral Artery (MCA) Doppler
The brain's response to hypoxia.
- Normal: High resistance (high PI) – brain isn't "stealing" blood.
- Low MCA PI ("Brain Sparing"): Cerebral vasodilation. Fetus is prioritizing brain perfusion. Sign of early hypoxia.
Cerebro-Placental Ratio (CPR)
MCA PI / UA PI
- Normal CPR: >1. MCA resistance higher than UA.
- Low CPR (<1): Red flag. Even if individual values seem normal, a low ratio suggests redistribution.
Ductus Venosus (DV) Doppler
Late-stage marker of cardiac compromise.
- Normal A-wave: Positive.
- Absent/Reversed A-wave: Myocardial dysfunction. Imminent fetal demise. Delivery indicated.
The Doppler Staging Model (Proposed)
| Stage | Findings | Frequency of Monitoring | Typical Delivery |
|---|---|---|---|
| I (Mild FGR) | AC <3rd or EFW <10th + Abnormal UA PI. | Weekly scans. | 37+ weeks. |
| II (Severe FGR) | AEDF or Low MCA PI (not both). | Twice weekly. | 34+ weeks. |
| III (Critical FGR) | REDF or DV abnormal. | Daily CTG. Admit. | Delivery ASAP. |
| IV (Terminal) | Abnormal CTG + all of above. | Continuous CTG. | Emergency CS. |
6. Investigations
Baseline Investigations (On Diagnosis of FGR)
| Investigation | Purpose |
|---|---|
| Full Blood Count | Check for HELLP syndrome. |
| LFTs | Pre-eclampsia screen. |
| Urate | Raised in Pre-eclampsia. |
| Urinalysis / PCR | Proteinuria (Pre-eclampsia). |
| Thrombophilia Screen | If severe/early FGR, consider Antiphospholipid Syndrome. |
| TORCH Serology | If structural abnormalities or severe symmetrical SGA (CMV, Toxoplasma, Rubella). |
Fetal Investigations
| Investigation | Purpose |
|---|---|
| Detailed Anatomy Scan | Rule out structural anomaly (especially cardiac). |
| Amniocentesis for Karyotype / Microarray | If early severe SGA or anomalies (Chromosomal cause: T18, T13). |
7. Management
Management Algorithm (RCOG Pathway)
┌─────────────────────────────────────────────────────────────────────┐
│ SGA DETECTED ON ULTRASOUND (<10th Centile) │
├─────────────────────────────────────────────────────────────────────┤
│ │
│ STEP 1: Is there a cause? │
│ ├── Screen for Pre-eclampsia (BP, Urine, Bloods). │
│ ├── Detailed Anatomy Scan. │
│ └── ?Karyotype if severe/early/anomalies. │
│ │
│ STEP 2: Check Umbilical Artery Doppler. │
│ │
│ ├── UA Doppler NORMAL ───────────────────────────────────────── │
│ │ ↓ │
│ │ Likely Constitutional SGA. │
│ │ Serial Growth Scans 2-4 weekly. Standard antenatal care. │
│ │ Plan delivery at 40+0 - 41+0 weeks. │
│ │ │
│ └── UA Doppler ABNORMAL (Raised PI, AEDF, REDF) ──────────────── │
│ ↓ │
│ FGR CONFIRMED. Increase Surveillance. │
│ │
│ STEP 3: Staging and Surveillance. │
│ ├── High PI only: Weekly scans. Deliver 37+0 weeks. │
│ ├── AEDF: Steroids. Twice weekly scans/CTG. Deliver 32-34 weeks. │
│ └── REDF / DV Abnormal: STEROIDS. Admit. Daily CTG. Delivery ASAP. │
│ │
│ STEP 4: Delivery. │
│ ├── If stable, await 32-34 weeks for steroids to work (if preterm).│
│ ├── Delivery by Caesarean Section often required. │
│ └── Neonatology involvement for preterm/compromised baby. │
│ │
└─────────────────────────────────────────────────────────────────────┘
Conservative Management (Surveillance)
| Doppler Finding | Surveillance Frequency | Delivery Gestation |
|---|---|---|
| SGA + Normal Dopplers | Growth scans 2-4 weekly. Standard antenatal. | 40+0 - 41+0 weeks. |
| High UA PI (no AEDF) | Weekly scans + Dopplers. | 37+0 weeks. |
| AEDF | Twice weekly scans + CTG. Admit if deterioration. | 32+0 - 34+0 weeks. May wait for steroids. |
| REDF | Daily CTG. Admit. Steroids. | ASAP once steroid benefit achieved (typically 48-72hrs post-steroids). |
| Abnormal DV / CTG | Continuous monitoring. | Delivery within hours. |
Pharmacological Management
- Steroids: Betamethasone 12mg IM x 2 doses 24 hours apart OR Dexamethasone 6mg IM x 4 doses 12 hours apart. Given if delivery anticipated <34+6 weeks.
- Magnesium Sulphate (MgSO4): For neuroprotection if delivery anticipated <32 weeks.
- Aspirin (Prophylaxis): 150mg OD from 12 weeks to 36 weeks prevents Pre-eclampsia and reduces SGA risk in high-risk women (ASPRE Trial).
Drill Down: Aspirin Prophylaxis for SGA Prevention
The ASPRE Revolution.
- Indication: High-risk women (identified at 1st trimester combined Pre-eclampsia screening OR by history: previous SGA, previous Pre-eclampsia, Chronic HTN).
- Dose: 150mg OD taken at night (not 75mg!). Evening dosing increases efficacy.
- Timing: Start at 12 weeks, continue to 36 weeks.
- Mechanism: Inhibits platelet Thromboxane A2 -> Improves placental blood flow -> Reduces risk of early-onset Pre-eclampsia and SGA.
- Evidence: ASPRE Trial showed 62% reduction in Preterm Pre-eclampsia.
- Important: Must be started <16 weeks for maximal benefit. Less effective if started later.
Drill Down: Magnesium Sulphate Neuroprotection
For the preterm brain.
- Indication: Delivery anticipated <32 weeks (regardless of mode).
- Dose: 4g IV loading dose (over 15-20 mins).
- Mechanism: Unknown but reduces risk of Cerebral Palsy.
- Side Effects: Flushing, Nausea. Maternal toxicity (loss of reflexes, respiratory depression) if overdose.
- NNT (Number Needed to Treat): ~60 to prevent 1 case of Cerebral Palsy.
Ethical Considerations: The Periviable Fetus
When FGR occurs at <24 weeks.
- The Dilemma: Prolonging pregnancy risks stillbirth. Delivering means extreme prematurity.
- Counselling: Honest discussion about survival rates and neurodevelopmental outcomes.
- Options: Active management vs. Palliative Care.
- MDT: Obstetrician + Neonatologist + Parents + Specialist Midwife.
Delivery Considerations
- Route: Vaginal delivery is possible if the fetus is tolerating labour. Continuous CTG is mandatory. Low threshold for Caesarean Section if abnormal CTG.
- Preterm FGR: Usually Elective CS due to poor fetal reserve.
8. Complications
Neonatal Complications (FGR Baby)
| Complication | Mechanism | Management |
|---|---|---|
| Hypoglycaemia | Depleted liver glycogen stores. | Early, frequent feeds. Check BM 2-4 hourly. IV Dextrose if severe. |
| Hypothermia | Low subcutaneous fat. | Skin-to-skin. Warmer. Monitor temp. |
| Polycythaemia | Chronic hypoxia stimulates EPO. | Monitor Haematocrit. Partial exchange transfusion if symptomatic. |
| NEC (Necrotising Enterocolitis) | Gut ischaemia from antenatal redistribution. | Careful feeding. Watch for abdominal distension. |
| Perinatal Asphyxia | If delivery delayed too long. | Resuscitation at birth. Neonatal cooling if HIE. |
| Long-term Neurodevelopmental Issues | Chronic hypoxia + Prematurity. | Follow-up, early intervention. |
Maternal Complications
- Emergency CS: High rate due to fetal compromise.
- Pre-eclampsia Progression: FGR often coexists with or precedes Pre-eclampsia.
Drill Down: Post-Natal Follow-Up (The FGR Baby)
Immediate to Long-term Surveillance.
Immediate (0-48 hours):
- Hypoglycaemia monitoring (BM at 2, 4, 6, 12, 24 hours).
- Hypothermia prevention (Skin-to-skin, Warmer).
- Early feeding (Breast or Formula, avoid prolonged fasting).
NICU/SCBU Indications:
- Severe SGA (<3rd centile) + Preterm.
- Respiratory distress.
- Hypoglycaemia refractory to feeding.
- Admission for polycythaemia monitoring.
Community/GP Follow-Up:
- Weight gain monitoring (May be slow initially then "catch-up" growth).
- Developmental surveillance (Higher risk of delay).
- Ophthalmology review if very preterm.
- Hearing screen.
Long-Term (The Barker Legacy):
- Increased risk of CVD, T2DM, Obesity in adulthood.
- Counsel parents on healthy lifestyle from childhood.
- Annual BP checks from young adulthood.
Quality Markers: Audit Standards (Saving Babies' Lives)
| Standard | Target |
|---|---|
| Women with risk factors for SGA identified at booking | >0% |
| High-risk women commenced on Aspirin <16 weeks | >0% |
| SFH measured and plotted at every antenatal visit | 100% |
| SGA suspicion triggers USS within 3 working days | >0% |
| Women with SGA managed according to RCOG GTG 31 pathway | 100% |
9. Prognosis & Outcomes
Fetal Outcomes
| Category | Outcome |
|---|---|
| Constitutional SGA | Generally excellent. No increased stillbirth risk. |
| FGR (Managed Well) | Good with appropriate surveillance and timely delivery. |
| Undetected FGR | Major contributor to stillbirth (Up to 50% of stillbirths have unrecognised FGR). |
| Severe Early-Onset FGR (<28 weeks) | Higher morbidity/mortality due to extreme prematurity at delivery. |
Long-Term Outcomes (Barker Hypothesis / DOHaD)
"Developmental Origins of Health and Disease."
- SGA babies, especially those exposed to undernutrition in utero, are "programmed" for metabolic efficiency.
- In later life, this manifests as:
- Increased risk of Obesity.
- Type 2 Diabetes.
- Hypertension.
- Cardiovascular Disease.
- Known as the "Thrifty Phenotype".
Special Populations: SGA in Multiple Pregnancy (Twins)
A unique challenge.
- Difference: Twins are often smaller than singletons at term. Population charts overestimate risk.
- Chorionicity Matters:
- MCDA (Monochorionic Diamniotic): High risk of TTTS and selective FGR.
- DCDA (Dichorionic Diamniotic): Lower risk. Treat each twin independently.
- Selective FGR (sFGR): When one twin is SGA and the other is not. Weight discordance >25% is concerning.
- Surveillance: Serial scans (fortnightly in MCDA from 16 weeks).
- Delivery Timing: Earlier than singletons (37w DCDA, 36w MCDA).
Special Populations: SGA in Pre-eclampsia
Shared pathway.
- FGR and Pre-eclampsia often coexist (Same underlying placental disease).
- Key Point: If you find SGA, always screen for Pre-eclampsia (BP, Proteinuria, LFTs, Platelets).
- Delivery Indication: May need earlier delivery for maternal reasons (severe HTN, HELLP) even if fetus is "coping".
Special Populations: Previous Stillbirth
The anxious pregnancy.
- Management: Consultant-led care. Serial growth scans from 26 weeks. Customised centiles. Aspirin 150mg from 12 weeks.
- Psychological Support: Previous stillbirth is traumatic. Additional scans for reassurance may be appropriate even if not strictly indicated. Bereavement midwife involvement.
- Delivery Planning: Often earlier IOL (37-38 weeks) even with normal findings, for parental anxiety and to reduce residual stillbirth risk.
Exam Scenarios (Common Vivas)
Scenario 1:
- Stem: 28-week scan shows EFW <5th centile. UA Doppler shows absent end diastolic flow. What do you do?
- Answer: Admit. Give Steroids (Betamethasone x2 doses). Consider MgSO4 neuroprotection. Daily CTG. Plan delivery ~48-72 hours post-steroids (32-34 weeks if stable). Neonatology counselling.
Scenario 2:
- Stem: 36-week scan shows EFW 8th centile. UA Doppler is normal. MCA is normal. What is the diagnosis and plan?
- Answer: Likely Constitutional SGA. Continue standard antenatal care. Plan delivery at 40-41 weeks.
Scenario 3:
- Stem: 32-week scan shows EFW 3rd centile, UA PI raised, MCA PI low ("brain sparing"). What next?
- Answer: FGR confirmed with fetal redistribution. Increase surveillance (twice weekly Dopplers/CTG). Consider delivery 34+ weeks. Steroids if not already given.
Scenario 4:
- Stem: A woman at her booking visit has a history of previous stillbirth attributed to placental insufficiency. What do you offer?
- Answer: High risk of recurrence. Aspirin 150mg OD from 12-36 weeks. Uterine Artery Doppler at 22-24 weeks. Serial growth scans from 26 weeks. Customised growth chart.
Scenario 5:
- Stem: SFH at 32 weeks is 28cm (well below the 3rd centile). What is your action?
- Answer: Refer for Ultrasound for EFW and Dopplers to confirm/exclude SGA/FGR.
Triage: When to Refer / Admit
| Scenario | Urgency | Action |
|---|---|---|
| SFH <3rd centile or static | Urgent | USS within 48 hours. |
| EFW <10th centile, Normal Dopplers | Routine | Standard antenatal, serial scans. |
| EFW <10th centile, Raised UA PI | Urgent | Weekly Dopplers. Consultant-led care. |
| AEDF (Absent End Diastolic Flow) | Emergency | Admit. Steroids. Twice daily CTG. Delivery plan 32-34w. |
| REDF (Reversed End Diastolic Flow) | Emergency | Admit. Steroids. Delivery often within 48-72 hours. |
| Abnormal DV / CTG | Critical | Continuous monitoring. Delivery ASAP. |
| Reduced Fetal Movements + SGA | Emergency | CTG and Doppler assessment same day. |
10. Evidence & Guidelines
Key Guidelines
| Guideline | Organisation | Year | Key Points |
|---|---|---|---|
| GTG 31: SGA Fetus | RCOG | 2013 (Updated 2014) | Risk factor screening. Doppler-based surveillance. Delivery timing. |
| Saving Babies' Lives Care Bundle v2 | NHS England | 2019 | Focus on SFH plotting, customised centiles, Carbon Monoxide monitoring. |
| ISUOG: FGR Guidelines | ISUOG | 2020 | Staging of FGR based on Dopplers (Figueras/Gratacos classification). |
Landmark Trials
1. TRUFFLE Study (2015) - Trial of Umbilical and Fetal Flow in Europe
- Question: What is the best parameter for timing delivery in early-onset FGR?
- Finding: Awaiting DV changes allowed pregnancy prolongation without increased adverse outcome compared to CTG-based timing.
- Impact: Supported the use of DV Doppler for delivery timing in severe preterm FGR.
2. ASPRE Trial (2017) - Aspirin for Evidence-based Pre-eclampsia Prevention
- Finding: Aspirin 150mg at night from 12-36 weeks reduced Pre-eclampsia by 62% in high-risk women.
- Impact: Also reduced SGA rates as Pre-eclampsia is a major cause.
11. Patient/Layperson Explanation
What does "Small for Gestational Age" mean?
It means your baby is measuring smaller than expected for how far along you are in your pregnancy. This is very common – about 1 in 10 babies measure this way.
Is my baby okay?
Many SGA babies are perfectly healthy; they are simply genetically small, often because their parents are small. However, some SGA babies are not growing properly because the placenta isn't working as well as it should. We need to do some extra scans and tests to check which group your baby is in.
What tests will I have?
- Ultrasound Scans: To measure your baby's size.
- Doppler Scans: A special type of ultrasound that looks at blood flow in the cord to see how well the placenta is working.
- CTG: A machine that monitors your baby's heart rate.
Will my baby need to be born early?
Some babies may need to be delivered early if the tests show they are not coping well in the womb. Your doctor will discuss the best timing with you. It's a balancing act between keeping baby inside (for growth) and getting them out (for safety).
What should I look out for?
Please monitor your baby's movements. If you notice a reduction in movements (baby moving less than usual), contact us immediately.
Key Counselling Points (For Clinicians)
- Explain the difference: "SGA means 'smaller than expected'. It doesn't always mean there's a problem."
- Explain the tests: "The Doppler scan checks blood flow in the cord – it tells us how well the placenta is working."
- Manage expectations: "We may need to deliver your baby earlier than planned to keep them safe."
- Fetal Movements: Reinforce the importance of monitoring movements and reporting any reduction immediately.
- Post-delivery: "Your baby may need extra monitoring after birth, but most SGA babies do very well."
12. References
- RCOG Green-top Guideline No. 31. The Investigation and Management of the Small–for–Gestational–Age Fetus. 2013. Link
- NHS England. Saving Babies' Lives Care Bundle Version Two. 2019. Link
- Lees CC, et al. TRUFFLE Study. Lancet. 2015. PMID: 25753280
- Rolnik DL, et al. ASPRE Trial. N Engl J Med. 2017. PMID: 28657417
Medical Disclaimer: MedVellum content is for educational purposes and clinical reference. Clinical decisions should account for individual patient circumstances. Always consult appropriate specialists for complex cases.