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Here is a thorough overview of Intrauterine Growth Restriction (IUGR) / Fetal Growth Restriction (FGR) drawn from authoritative textbooks.
Intrauterine Growth Restriction (IUGR / FGR)
Definition
IUGR (now increasingly called FGR - Fetal Growth Restriction) refers to a fetus that fails to achieve its growth potential. The most widely used threshold in the United States (ACOG) is an estimated fetal weight (EFW) or abdominal circumference (AC) below the 10th percentile for gestational age on ultrasound. However, most adverse perinatal outcomes are concentrated below the 5th or 3rd percentile.
- SGA (small for gestational age): Applied to the newborn (weight <10th percentile for GA). Not synonymous with FGR - some SGA infants are constitutionally small and healthy.
- FGR: Implies the fetus is not reaching its growth potential due to a pathological process.
Classification: Symmetric vs. Asymmetric IUGR
| Feature | Symmetric IUGR | Asymmetric IUGR ("Head-Sparing") |
|---|---|---|
| All organs equally reduced | Yes | No - head/brain spared |
| Timing | Early onset, persistent or severe insult | Later, gradual compromise |
| Abdominal circumference | Reduced proportionally | Reduced first (small liver) |
| HC/AC ratio | Normal (~1.0) | Elevated (>0.95 at term) |
| Typical causes | Chromosomal abnormalities, TORCH infections, fetal genetic disorders | Placental insufficiency, maternal vascular disease, malnutrition |
In asymmetric FGR, fetal autoregulation shunts blood preferentially to the brain and adrenal glands, at the expense of kidneys (causing oligohydramnios) and viscera.
- Textbook of Family Medicine 9e | Robbins & Kumar Basic Pathology
Etiology / Causes
1. Maternal Factors (most common)
- Vascular disease: preeclampsia, chronic hypertension
- Hypercoagulability: antiphospholipid syndrome, inherited thrombophilias
- Malnutrition, prolonged hypoglycemia
- Substance use: heavy cigarette smoking, narcotic use, alcohol
- Drugs: teratogenic (phenytoin) and non-teratogenic agents
- Connective tissue disorders
- Diabetes with vascular involvement
2. Fetal Factors
- Chromosomal disorders (trisomies - especially trisomy 18, 13)
- Congenital anomalies
- TORCH infections (Toxoplasma, Rubella, CMV, Herpes/HIV) - a common cause of symmetric FGR
- Multiple gestation
3. Placental Factors
- Placenta previa, placental abruption, placental infarction
- Chronic villitis, hemorrhagic endovasculitis
- Placental mosaicism
- Any factor that compromises uteroplacental blood flow
The etiology of FGR is unknown in approximately 60% of cases.
- Creasy & Resnik's Maternal-Fetal Medicine
Pathophysiology
In FGR caused by placental insufficiency:
- Reduced terminal villi growth decreases the umbilical capillary bed
- Umbilical blood flow per kg fetal weight is reduced (pulsatility index elevated)
- Placental oxygen permeability falls, causing fetal hypoxemia (umbilical venous PO2 ~12 mmHg below normal; O2 saturation drops from 81% to ~50%)
- Transplacental glucose permeability also decreases
- Fetal growth slows as an adaptive response to reduce oxygen demand
- Creasy & Resnik's Maternal-Fetal Medicine
Diagnosis
Clinical Screening
- Fundal height (in cm, 24-38 weeks) approximates gestational age; measurements lagging by 3+ cm warrant ultrasound
- A single fundal height at 32-34 weeks: ~85% sensitive, ~96% specific for FGR
Ultrasound Diagnosis (80-90% sensitive)
Key measurements:
- Abdominal circumference (AC) - first parameter to fall behind (reflects liver glycogen stores)
- Biparietal diameter (BPD), head circumference (HC), femur length (FL)
- EFW plotted on growth curves
- HC/AC ratio: >0.95 (>1.0 after 36 weeks) detects ~85% of asymmetric IUGR
- FL/AC ratio: ≥23.5% suggests asymmetric IUGR
Doppler Velocimetry (Most Valuable Surveillance Tool)
Doppler assesses placental resistance through changes in umbilical artery flow:
| Doppler Finding | Clinical Significance |
|---|---|
| Decreased end-diastolic flow | Early sign; rarely causes significant neonatal morbidity alone |
| Absent end-diastolic flow (AEDF) | Significant - high perinatal morbidity/mortality risk |
| Reversed end-diastolic flow (REDF) | Most severe; 5x higher mortality than AEDF |
| Absent/reversed ductus venosus a-wave | Ominous - impending acidemia or death, usually within 7 days |
Progression in severe FGR: umbilical artery changes → middle cerebral artery changes (increased diastolic flow = "brain-sparing") → ductus venosus/umbilical vein venous changes.
Umbilical artery Doppler reduces perinatal mortality by ~29% when added to antenatal surveillance.
Other Tests
- Nonstress test (NST) and biophysical profile (BPP) for fetal well-being
- Cordocentesis (fetal blood sampling) - can directly measure PO2, pH, hemoglobin
- Amniocentesis - karyotype if chromosomal cause suspected
Management
Antenatal
- Bed rest in lateral recumbent position - prevents vena caval compression, maximizes cardiac output and uteroplacental perfusion
- Treat underlying maternal condition (e.g., optimize hypertension control)
- Serial ultrasound every 2-3 weeks to assess growth velocity
- Regular antenatal testing: NST, BPP, Doppler velocimetry
- Corticosteroids if delivery is anticipated before 34 weeks (for fetal lung maturity)
Timing of Delivery
| Gestational Age | Doppler Finding | Action |
|---|---|---|
| ≥34 weeks | Absent end-diastolic flow | Deliver |
| ≥32 weeks | Reversed end-diastolic flow | Deliver |
| <32 weeks, reversed flow | Absent/reversed flow | Consider extended monitoring (BPP, CTG, cerebroplacental ratio) with attempt to prolong pregnancy; deliver if ductus venosus abnormal |
| Idiopathic FGR, >34 weeks | Any | Provider-initiated delivery to prevent stillbirth |
| Term (38-40 weeks) | Mild restriction | Deliver, depending on degree |
After 34 weeks, in idiopathic FGR, delivery is indicated to avoid stillbirth. Before 34 weeks, maternal status or Doppler deterioration guides timing.
Mode of Delivery
- Severely growth-restricted fetuses often delivered by cesarean section as they are unlikely to tolerate labor stress
- Trial of labor may be considered with a favorable cervix in less severe cases
Complications / Associations
Perinatal:
- Fetal distress, non-reassuring fetal heart rate
- Intrauterine fetal demise (stillbirth)
- Low Apgar scores
- Hypothermia, hypoglycemia, polycythemia
- Sepsis, necrotizing enterocolitis
- Accelerated fetal lung maturity (relative "benefit")
- Meconium aspiration
Long-term:
- Cerebral dysfunction, learning disabilities
- Visual and hearing impairment
- Increased risk of cardiovascular disease, type 2 diabetes, hypertension in adult life (Barker hypothesis / developmental origins of health and disease)
Note: Obesity is NOT commonly associated with IUGR (it is associated with macrosomia instead).
Key Points to Remember
- IUGR = EFW or AC <10th percentile; most morbidity concentrated below 3rd-5th percentile
- Asymmetric (head-sparing) FGR is caused by late/chronic placental insufficiency; symmetric FGR by early or intrinsic fetal insults
- AC is the first ultrasound parameter to fall behind
- Umbilical artery Doppler is the most valuable management tool - not a screening tool
- Reversed end-diastolic flow is 5x more lethal than absent end-diastolic flow
- Absent/reversed ductus venosus a-wave = deliver imminently
- Deliver at ≥34 weeks if absent flow; ≥32 weeks if reversed flow
- Corticosteroids for lung maturity if <34 weeks and delivery anticipated
Sources:
- Creasy & Resnik's Maternal-Fetal Medicine: Principles and Practice
- Textbook of Family Medicine, 9th Ed.
- Robbins & Kumar Basic Pathology
- Swanson's Family Medicine Review
- Pfenninger and Fowler's Procedures for Primary Care
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