A 28-year-old primigravida at 32 weeks of gestation presents for a routine antenatal visit. Ultrasonography reveals an estimated fetal weight below the 10th percentile for gestational age with normal amniotic fluid volume and normal umbilical artery Doppler. Which is the most common cause of intrauterine growth restriction in this clinical scenario?

Explanation

## Most Common Cause of IUGR ### Epidemiology of IUGR Causes **Key Point:** Placental insufficiency secondary to uteroplacental malperfusion is the single most common cause of IUGR, accounting for approximately 70–80% of all cases of pathological growth restriction. ### Pathophysiology Placental insufficiency occurs due to: 1. Inadequate trophoblastic invasion of spiral arteries 2. Failure of physiological transformation of maternal vessels 3. Reduced placental perfusion and oxygen delivery 4. Chronic fetal hypoxia leading to growth restriction ### Classification of IUGR | Feature | Placental (Type I) | Constitutional (Type II) | |---------|-------------------|-------------------------| | **Frequency** | 70–80% | 20–30% | | **Onset** | Third trimester (often) | Early, stable | | **Amniotic fluid** | Often reduced | Normal | | **Doppler findings** | Abnormal (elevated PI, RI) | Normal | | **Perinatal morbidity** | High | Low | | **Mechanism** | Placental insufficiency | Genetic/constitutional | ### Clinical Context in This Case **High-Yield:** The normal amniotic fluid volume and normal umbilical artery Doppler in this case suggest **constitutional IUGR** (small-for-dates but constitutionally normal fetus). However, the question asks for the *most common cause overall* in the general IUGR population, not the most likely diagnosis in this specific case. **Clinical Pearl:** In clinical practice, when a fetus is small but has reassuring Doppler studies and normal liquor, it is often a constitutionally small but healthy infant. However, placental insufficiency remains the most common pathological cause of IUGR across all pregnancies. ### Why Placental Insufficiency Is Most Common - Accounts for the majority of IUGR cases requiring intervention - Associated with maternal vascular disease (hypertension, preeclampsia, diabetes) - Leads to abnormal Doppler patterns (elevated umbilical artery PI/RI, reversed end-diastolic flow in severe cases) - Responsible for most perinatal morbidity and mortality in IUGR **Warning:** Do not confuse the most common cause of IUGR (placental insufficiency) with the most common cause of small-for-gestational-age (SGA) infants, which includes constitutional smallness as a major contributor.