Clinical overview
Fetal growth restriction (FGR), or fetal growth restriction (FGR), describes a fetus that has failed to reach its genetically determined growth potential, usually because of placental insufficiency. It is the single most important antenatal antecedent of stillbirth, and a major contributor to perinatal mortality and to long-term neurodevelopmental and cardiometabolic morbidity. In South Africa, where obstetric haemorrhage, hypertension and HIV-associated infection dominate maternal deaths, FGR is the quiet engine behind a large share of the fetal and early-neonatal deaths counted in perinatal audit — many of them avoidable through earlier detection and timely delivery.
The single most useful conceptual move a registrar can make is to stop thinking of FGR as one disease and instead split it into early-onset (diagnosed before 32 weeks) and late-onset (at or after 32 weeks) phenotypes. They share a cause — placental insufficiency — but diverge in severity, the order in which Doppler abnormalities appear, the speed of deterioration, the surveillance tool that detects them, and the obstetric decision that follows. Early-onset FGR is severe, deteriorates predictably over days to weeks, and is dominated by the conflict between prematurity and progressive hypoxia. Late-onset FGR is milder per fetus but vastly commoner, deteriorates fast and unpredictably, is easy to miss because the fetus is not small, and is responsible for a disproportionate share of unexplained term stillbirth. Distinguishing the two — and applying the correct surveillance and delivery logic to each — is the HOTS skill this objective tests.
The first essential distinction is between a small-for-gestational-age (SGA) fetus that is constitutionally small but healthy, and a growth-restricted fetus that is pathologically failing to grow. Not every small fetus is compromised, and — critically — not every compromised fetus is small. This is why modern definitions combine size with functional (Doppler) and growth-trajectory criteria rather than relying on a single estimated-weight centile.
Figure J21.1 — Early- and late-onset FGR are distinct placental-insufficiency phenotypes separated at 32 weeks, with different Doppler clues and timing risks.
Core knowledge
Definitions
- SGA: estimated fetal weight (EFW) or abdominal circumference (AC) below the 10th centile. A statistical, not a pathological, label.
- FGR: failure to achieve growth potential, usually evidenced by EFW/AC below a low centile combined with Doppler abnormality and/or slowing of growth across serial scans. The widely used Delphi consensus definition of FGR (referenced in RCOG GTG 31) separates early from late forms at 32 weeks and grades severity by EFW/AC centile (e.g. <3rd centile as a stand-alone criterion) plus umbilical-artery, uterine-artery and middle-cerebral-artery Doppler thresholds. Quote the framework, not invented cut-offs.
- Symmetrical vs asymmetrical: an older but still useful heuristic. Asymmetrical restriction (head-sparing, AC reduced more than head circumference) suggests placental insufficiency of later onset, where the fetus redistributes cardiac output to the brain. Symmetrical restriction (head and body equally small) is more typical of an early insult — aneuploidy, congenital infection or very early severe placental disease. The dichotomy is imperfect and should not override Doppler-based phenotyping.
Shared cause, divergent biology
Both phenotypes usually stem from placental insufficiency — see Placental insufficiency response for the fetal physiology. The placenta fails to deliver enough oxygen and substrate, and the fetus mounts an adaptive response that ends, if unrelieved, in hypoxia, acidaemia and death.
Early-onset FGR reflects severe, often global, placental maldevelopment — failure of trophoblast invasion and spiral-artery remodelling, high-resistance uteroplacental flow, and frequently shared pathophysiology with pre-eclampsia (the two coexist commonly because both arise from defective placentation; see Pre eclampsia and HELLP and Hypertension in pregnancy). A large proportion of the villous tree is affected, so the umbilical artery — which integrates resistance across the whole placenta — shows abnormality early. The hallmark Doppler cascade is therefore detectable and sequential:
- Rising umbilical-artery (UA) resistance (raised PI), then
- Absent end-diastolic flow (AEDF), then
- Reversed end-diastolic flow (REDF) in the UA;
- Middle-cerebral-artery (MCA) vasodilation (falling PI) — "brain-sparing" — and a falling cerebroplacental ratio (CPR);
- Late, ominous deterioration of ductus venosus (DV) waveforms (absent or reversed a-wave) and ultimately abnormal CTG / biophysical profile.
This orderly progression is what makes early-onset FGR manageable: it gives the clinician a staged warning system over days to weeks within which to optimise neonatal readiness (corticosteroids, magnesium sulphate, transfer to an appropriate level of care) before delivering.