Fetal medicine · Surveillance imaging

IUGR and Doppler studies — what the waveforms mean and when to deliver

Intra-uterine growth restriction (IUGR) is a pregnancy in which the fetus has not achieved its growth potential. Distinguishing constitutional small-for-gestational-age (SGA) from true placental insufficiency, and timing delivery correctly, requires structured Doppler surveillance. This page describes how Doppler studies are used at this centre — the vessels examined, what each waveform tells us, and the staged decision-making about delivery.

IUGR versus SGA

Two terms are used and they are not the same:

• SGA (small for gestational age) — estimated fetal weight (EFW) below the 10th centile for gestational age. A statistical definition. Many SGA babies are simply constitutionally small and healthy.
• IUGR (intra-uterine growth restriction) — failure of the fetus to reach its biological growth potential, usually due to placental insufficiency. Identified by EFW below the 3rd centile, OR EFW below the 10th centile with abnormal Doppler or growth velocity.

The Doppler studies are what separate the two clinically.

The Doppler vessels

• Umbilical artery (UA) — reflects placental resistance. Earliest abnormal sign. Pulsatility index (PI) above the 95th centile, then absent end-diastolic flow (AEDF), then reversed end-diastolic flow (REDF) as deterioration progresses.
• Middle cerebral artery (MCA) — reflects fetal brain perfusion. PI below the 5th centile suggests fetal “brain-sparing” — a compensatory redistribution of blood to the brain in chronic hypoxia.
• Cerebroplacental ratio (CPR) — MCA PI divided by UA PI. CPR below the 5th centile is an integrated marker of fetal compromise.
• Ductus venosus (DV) — reflects right-heart afterload. Absent or reversed a-wave is a late, ominous sign indicating critical fetal compromise.
• Uterine artery — maternal placental flow. Used in early pregnancy for pre-eclampsia and IUGR risk prediction.

Staged interpretation

1. Stage 1 (early FGR) — SGA + abnormal UA PI or CPR. Fortnightly surveillance.
2. Stage 2 — UA AEDF. Weekly surveillance, steroids, plan delivery at 34 weeks.
3. Stage 3 — UA REDF or DV-PI >95th centile. Bi-weekly surveillance, plan delivery at 30–32 weeks.
4. Stage 4 — DV a-wave absent/reversed, abnormal cardiotocograph (CTG), or biophysical profile abnormality. Deliver at 26–30 weeks per balance of risks.

Delivery timing

Delivery is timed against three competing risks: fetal compromise if pregnancy continues, prematurity if delivery is too early, and the iatrogenic risks of intervention. ISUOG and the TRUFFLE trial findings inform the decision matrix. The trade-off is reassessed at each surveillance scan.

Mode of delivery

Mode of delivery is individualised. Vaginal birth is possible in early FGR where the fetus tolerates labour. Caesarean is preferred in advanced FGR with severe Doppler abnormality, AEDF/REDF, or non-reassuring CTG. Continuous intrapartum CTG monitoring is standard for any FGR pregnancy in labour.

After delivery

FGR babies need neonatal team involvement at birth. Long-term follow-up of growth, neurodevelopment, and cardiometabolic risk follows. The postnatal conversation with the couple includes the placental histology findings and the implications (if any) for subsequent pregnancies.

When to refer

• Any pregnancy where the growth scan suggests EFW below the 10th centile
• Any pregnancy with abnormal Doppler — even if EFW is normal
• Significant deceleration in growth velocity (centile crossing of more than 50 centiles between scans)
• Past pregnancy history with FGR or stillbirth

Guidelines we follow on this topic

• ISUOG Practice Guideline on Diagnosis and Management of Small-for-Gestational-Age Fetus and Fetal Growth Restriction
• RCOG Green-top on the Investigation and Management of the Small-for-Gestational-Age Fetus
• SMFM Consult Series on FGR
• TRUFFLE trial findings

Related reading

• Fetal medicine programme
• Fetal medicine scans
• Fetal medicine conditions
• Pregnancy care
• High-risk pregnancy
• Menstrual cycle physiology

Dr. Priyadatt Patel

Senior Gynecologist · Advanced Laparoscopic Surgeon · IVF and Endometriosis Programme Lead

MS OBGyn · Pregnancy Care · Advanced Gynaecological Ultrasound · Fertility Preservation

ESHRE / ESGE / AAGL / ASRM guideline-aligned practice. 3D Karl Storz precision technique. Fertility-preservation-first philosophy. Evidence-based decisions, honest counselling, long-term outcomes orientation.