Preparatory Mindset
Fetal growth and amniotic fluid are the "fetal well-being" parameters of antenatal care. The exam mindset: a small-for-dates uterus (fundal height lag) triggers the FGR workup, and abnormal amniotic fluid volume (oligohydramnios / polyhydramnios) is a red flag for fetal anomalies, placental dysfunction, or maternal disease. Key distinction to master: FGR (fetal growth restriction) = pathological; SGA (small for gestational age) = constitutionally small but healthy — treatment and prognosis differ. For multifetal pregnancy, the twin-specific complications (TTTS, growth discordance, preterm birth) are the high-yield items. The unifying principle: detect the growth problem, screen for the cause, monitor serially, and time delivery to balance hypoxia risk vs prematurity.
Core Concepts
1. Fetal growth — definitions
| Term | Definition |
|---|---|
| FGR (fetal growth restriction / IUGR) | EFW (estimated fetal weight) < 10th percentile for gestational age with evidence of pathology (abnormal Doppler, oligohydramnios, falling growth velocity, abnormal biometry) |
| SGA | EFW < 10th percentile but constitutionally normal (healthy small baby, normal Doppler/fluid/velocity) |
| LBW | Birth weight < 2500 g (includes preterm) |
Morphology:
- Symmetric FGR (~20%): all biometrics small — early insult (chromosomal, congenital infection, early-onset) — poorer prognosis.
- Asymmetric FGR (~80%): head spared, abdomen small (brain-sparing) — late placental insufficiency (preeclampsia, chronic hypoxia) — better catch-up potential.
2. Aetiology of FGR
| Category | Examples |
|---|---|
| Fetal | Chromosomal abnormalities, congenital anomalies, infection (TORCH — CMV, toxoplasma, rubella), multiple pregnancy |
| Maternal | Preeclampsia/chronic hypertension, renal disease, diabetes with vasculopathy, malnutrition, smoking, alcohol, drugs, anaemia |
| Placental | Abnormal placentation, infarction, umbilical cord anomalies (single umbilical artery, velamentous insertion, cord twining) |
3. Screening and diagnosis of FGR
- Umbilical artery Doppler — absent/reversed end-diastolic flow (AEDF/REDF) = severe placental insufficiency — urgent delivery decision. - MCA Doppler — brain sparing (low MCA PI). - Cerebroplacental ratio (CPR) — <1 = redistributed flow. - Ductus venosus — abnormal = deteriorating fetal status.
- Fundal height measurement at each visit (lag >2 cm → scan).
- USS biometry: BPD, HC, AC, FL → EFW percentile; serial scans (2-3 weekly) show growth velocity.
- Doppler studies (the key prognostic tool):
- Amniotic fluid — oligohydramnios often coexists (placental dysfunction).
Workup on diagnosis: detailed anomaly scan, infection screen (TORCH), karyotype (if structural anomaly/symmetric early FGR), maternal disease screen (BP, urine, glucose, renal), growth re-assessment.
4. Management of FGR
| Component | Detail |
|---|---|
| Monitoring | Serial USS (growth + Dopplers + AFI) every 2-3 weeks; antenatal CTG/NST; biophysical profile; kick charts |
| Optimise | Treat maternal cause (BP control, stop smoking/alcohol, nutrition); rest |
| Delivery timing | No fetal compromise: deliver ≥37-38 wk (term FGR) or ≥34 wk if stable early-onset; abnormal Doppler (AEDF) → consider delivery ≥34 wk after steroids; REDF / abnormal ductus venosus / pathological CTG → deliver immediately (after steroids if <34 wk) |
| Mode | Vaginal if no compromise; C-section for fetal distress / malpresentation / severe compromise |
| Low-dose aspirin | For prevention in high-risk women (preeclampsia/FGR risk) — started <16 wk |
5. Abnormal amniotic fluid volume
| Condition | Definition | Causes | Management |
|---|---|---|---|
| Oligohydramnios | AFI <5 cm / deepest pocket <2 cm | Fetal (renal agenesis, posterior urethral valves, PROM), placental insufficiency, post-term, maternal (NSAIDs) | Identify cause; serial monitoring; amnioinfusion not routine; delivery if term + fetal compromise |
| Polyhydramnios | AFI >24 cm / deepest pocket >8 cm | Fetal (anencephaly, oesophageal atresia, duodenal atresia, neural tube defect), maternal diabetes (hyperglycaemia → fetal polyuria), multifetal, isoimmunisation | Anomaly scan + OGTT; therapeutic amnio for severe symptoms; NSAIDs (indomethacin) in selected cases; monitor for preterm labour |
6. Multifetal pregnancy — essentials
- Types: Dizygotic (2 eggs, ~70%) vs monozygotic (1 egg, ~30%); chorionicity determined by USS (lambda/T sign) — monochorionic twins carry the highest risk.
- Maternal risks: preeclampsia (2-3×), GDM, anaemia, polyhydramnios, preterm labour, APH (vasa praevia, placenta praevia).
- Fetal risks: preterm birth (biggest risk — mean GA ~36 wk for twins), discordant growth (FGR in one twin), congenital anomalies, TTTS (twin-twin transfusion syndrome — monochorionic, shared placenta, unequal shunting → recipient polyhydramnios/plethora, donor oligohydramnios/growth restriction), twin anaemia-polycythaemia sequence, cord entanglement (monoamniotic), locked twins.
- Management: serial growth scans (every 2-4 wk), preterm birth prevention (cervical length, progesterone), antenatal corticosteroids if preterm labour, delivery planning (twin A vertex → attempt vaginal; otherwise C-section); TTTS → fetoscopic laser photocoagulation or amniodrainage.
High-Yield Points
| Topic | Must-remember |
|---|---|
| FGR vs SGA | FGR = <10th percentile + pathology (Doppler/fluid/velocity); SGA = healthy small |
| Symmetric vs asymmetric | Early insult (symmetric, ~20%) vs late placental insufficiency (asymmetric, ~80%) |
| Key Doppler | Umbilical AEDF/REDF = severe — delivery decision point |
| Brain sparing | MCA PI low / CPR <1 = redistribution |
| Oligohydramnios | AFI <5 cm — think renal anomalies / PROM / placental insufficiency |
| Polyhydramnios | AFI >24 cm — think GI obstruction, anencephaly, GDM, TTTS recipient |
| Multifetal biggest risk | Preterm birth |
| TTTS | Monochorionic only; recipient poly/plethora, donor oligo/Restricted; laser treatment |
| Aspirin prevention | Low-dose aspirin <16 wk for preeclampsia/FGR risk |
| FGR delivery | Stable term FGR → 37-38 wk; AEDF → ≥34 wk after steroids; REDF/abnormal DV → immediate |
Topic Summary
FGR is pathological smallness (<10th percentile + abnormal Doppler/fluid/velocity); symmetric = early insult, asymmetric = placental insufficiency. Diagnosis by fundal height + USS biometry + umbilical artery Doppler (AEDF/REDF = severe); management = serial monitoring + delivery timing based on Doppler and gestational age. Oligohydramnios (renal/PROM/placental) and polyhydramnios (GI obstruction, GDM, TTTS) flag fetal and maternal disease. Multifetal pregnancy carries preterm birth as its main risk, with monochorionic twins at risk of TTTS (laser treatment). The shared principle: screen, diagnose the cause, monitor serially, time delivery deliberately.
LMCHK OSCE Practice — Small Baby — FGR Counselling and Delivery Plan
Station setup: A 31-year-old primigravida at 33 weeks with chronic hypertension presents for USS: EFW 5th percentile, umbilical artery Doppler with absent end-diastolic flow (AEDF), AFI 4 cm, normal BPP 8/8, CTG reactive. BP 150/95 on labetalol.
Candidate tasks (8 min):
- Interpret the USS — FGR (EFW <10th + AEDF + oligohydramnios) — placental insufficiency likely.
- Explain the significance of AEDF (severe placental insufficiency — fetal hypoxia risk) and oligohydramnios.
- Outline the management plan: admit, antenatal corticosteroids (betamethasone) for lung maturity, serial CTG + Dopplers, BP control, plan delivery at ≥34 weeks (or sooner if decompensation — REDF, abnormal DV, pathological CTG).
- Discuss mode of delivery (C-section likely given FGR + AEDF) and neonatal care (NICU for preterm).
- Counsel on prevention for next pregnancy (aspirin, BP control).
Key marking cues:
- Recognises AEDF = severe, delivery decision point.
- Gives betamethasone before delivery.
- Balances hypoxia vs prematurity in delivery timing.
- Explains neonatal implications (preterm, NICU).
- Addresses BP control and aspirin for future pregnancy.