Intrauterine Growth Restriction (IUGR) / Fetal Growth Restriction (FGR)

Definition

IUGR (now more commonly termed Fetal Growth Restriction, FGR) refers to a fetus that fails to achieve its genetically predetermined growth potential, typically defined as estimated fetal weight or birth weight below the 10th percentile for gestational age, with severe FGR being below the 3rd percentile. It's distinct from but overlapping with "small for gestational age" (SGA) - SGA is purely a statistical definition based on size, while FGR implies a pathological process limiting growth (- Robbins Pathologic Basis of Disease; - Creasy & Resnik's Maternal-Fetal Medicine).

Classification

Symmetric FGR: All organs (head, abdomen, length) proportionally affected. Usually reflects an insult acting early in gestation - chromosomal disorders, congenital anomalies, congenital infections (TORCH). Growth restriction is intrinsic to the fetus.
Asymmetric FGR: Brain growth is relatively spared while abdominal/visceral organ growth (especially liver) lags. Reflects late-onset uteroplacental insufficiency where the fetus redistributes blood flow to protect the brain ("brain-sparing effect"). (- Robbins & Kumar Basic Pathology, p. 137)

Causes

Maternal factors (most common category):

Vascular disease: preeclampsia, chronic hypertension
Hypercoagulable states (acquired or inherited)
Substance use: smoking, alcohol, narcotics
Teratogenic/nonteratogenic drugs (e.g., phenytoin)
Malnutrition, prolonged hypoglycemia

Fetal factors: chromosomal disorders, congenital anomalies, congenital infections (TORCH) - these tend to cause symmetric FGR.

Placental factors: placenta previa, placental abruption, placental infarction, reduced terminal villi growth reducing umbilical blood flow and placental oxygen/glucose permeability - these tend to cause asymmetric FGR.

Pathophysiology (placental insufficiency mechanism)

Reduced terminal villous growth reduces umbilical blood flow per kilogram of fetal weight (reflected in elevated umbilical artery pulsatility index), decreases placental oxygen permeability (leading to fetal hypoxemia - PO2 roughly 12 torr below normal, oxygen saturation dropping from ~81% to ~50%), and reduces transplacental glucose diffusion. The fetus compensates by slowing growth to reduce oxygen demand (- Creasy & Resnik's Maternal-Fetal Medicine, Ch. 14).

Diagnosis/Monitoring

Serial ultrasound biometry (abdominal circumference most sensitive single measure)
Umbilical artery Doppler (pulsatility index, absent/reversed end-diastolic flow signals worsening placental insufficiency)
Middle cerebral artery Doppler (assesses brain-sparing)
Amniotic fluid volume (oligohydramnios common)
Biophysical profile / non-stress testing for surveillance

Complications

Perinatal: hypoxia, stillbirth risk, need for early delivery, neonatal hypoglycemia, temperature instability, polycythemia
Long-term: cerebral dysfunction, learning disabilities, sensory impairment, and increased risk of adult metabolic disease (Barker hypothesis - links to later cardiovascular/metabolic disease)

Recent guideline evidence

Current practice guidance (2023 SOGC Guideline No. 442, and a 2025 expert review in the Journal of Maternal-Fetal & Neonatal Medicine) emphasizes Doppler-based surveillance protocols and timing of delivery based on Doppler deterioration rather than growth percentile alone - consistent with the textbook physiology above, so no contradiction with the library content (PMID: 37730302, PMID: 40623849).

This is a broad topic — I gave you the core overview. If you want, I can go deeper into any of these specific areas:

Diagnostic workup and Doppler surveillance protocols (staging by MCA/UA/DV Doppler)
Management and timing of delivery (SOGC/ISUOG guideline recommendations)
Distinguishing FGR from constitutionally small (SGA) fetuses
Long-term outcomes and Barker hypothesis (fetal origins of adult disease)
Specific causes (e.g., preeclampsia-related placental insufficiency, TORCH infections, chromosomal causes)

Let me know which one you'd like, or ask a more specific question.

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