In a 36‑week‑gestation fetus with intrauterine growth restriction, what are the differential diagnoses and which maternal history questions should be asked?

Differential Diagnosis and Key History Questions for IUGR at 36 Weeks

At 36 weeks gestation with IUGR, you must systematically distinguish between three main categories: constitutionally small fetus, placenta-mediated growth restriction, and non-placental causes (fetal anomalies, infections, chromosomal abnormalities). 1

Differential Diagnosis Framework

Placenta-Mediated FGR (Most Common at 36 Weeks)

This late-onset presentation typically involves milder placental dysfunction compared to early-onset disease. The key differentiating features include:

• Maternal vascular disease: Pre-eclampsia, chronic hypertension, renal disease, autoimmune conditions (especially antiphospholipid syndrome), thrombophilias
• Placental insufficiency: Abnormal placental implantation or vasculature affecting nutrient/oxygen transfer
• Maternal medical conditions: Diabetes with vasculopathy, cardiac disease, severe anemia 1

Constitutionally Small (Normal Variant)

• Fetus tracking along lower percentiles but maintaining growth velocity
• Normal umbilical artery Doppler
• No maternal risk factors
• Appropriate amniotic fluid volume

Non-Placental Causes

At 36 weeks, these are less likely if not previously detected, but consider:

• Chromosomal abnormalities: Trisomy 13,18,21, Turner syndrome
• Congenital infections: Cytomegalovirus (most relevant), toxoplasmosis (though screening not recommended without risk factors) 2
• Structural anomalies: Cardiac defects, renal anomalies, skeletal dysplasias
• Genetic syndromes: Account for approximately 20% of FGR cases 2

Essential History Questions

Maternal Medical History

• Hypertensive disorders: "Do you have chronic high blood pressure or has your blood pressure been elevated during this pregnancy?"
• Autoimmune disease: "Have you been diagnosed with lupus, antiphospholipid syndrome, or other autoimmune conditions?"
• Renal disease: "Do you have any kidney problems or protein in your urine?"
• Diabetes: "Do you have diabetes, and if so, how long have you had it?" (duration matters for vascular complications)
• Cardiac disease: "Do you have any heart conditions?"
• Thrombophilia: "Have you had blood clots or been told you have a clotting disorder?"

Nutritional and Lifestyle Factors

• Pre-pregnancy weight: "What was your weight before pregnancy?" (low BMI is a risk factor) 3
• Nutrition: "Have you had adequate food intake? Any eating disorders or severe dietary restrictions?"
• Substance use: "Do you smoke, use alcohol, or use any recreational drugs?" (critical risk factors)
• Medications: "What medications are you taking?" (some can affect fetal growth)

Obstetric History

• Previous IUGR: "Have you had a small baby before or been told a previous baby wasn't growing well?"
• Previous stillbirth: "Have you had a pregnancy loss after 20 weeks?"
• Previous pre-eclampsia: "Did you have high blood pressure or protein in urine in previous pregnancies?"
• Dating accuracy: "When was your last menstrual period? Did you have an early ultrasound?" (essential to confirm gestational age)

Current Pregnancy

• Fetal movement: "Have you noticed decreased fetal movement?" (may indicate fetal compromise)
• Symptoms of pre-eclampsia: "Do you have headaches, vision changes, right upper abdominal pain, or sudden swelling?"
• Bleeding or fluid loss: "Have you had any vaginal bleeding or leaking fluid?"
• Growth trajectory: Review serial ultrasounds to determine if growth velocity has declined (crossing centiles) 1

Infection Risk Assessment

• CMV exposure: "Do you work in childcare or have young children at home? Any recent viral illness with fever?"
• Travel history: "Have you traveled to areas with Zika or other infections?"
• Rash or fever: "Have you had any rashes or fevers during pregnancy?"

Note: Routine screening for toxoplasmosis, rubella, or herpes is NOT recommended in IUGR without specific risk factors. However, CMV PCR should be considered if amniocentesis is performed for other indications. 2

Critical Clinical Pitfalls

Avoid assuming all small fetuses are pathologic. At 36 weeks with normal Doppler studies and appropriate growth velocity, many represent constitutional variation rather than true restriction 1.

Don't miss dating errors. Inaccurate gestational age is a common cause of falsely diagnosed IUGR. Confirm dating with first-trimester ultrasound measurements.

Recognize that late-onset FGR (like at 36 weeks) has different pathophysiology than early-onset disease. These fetuses typically have milder placental dysfunction, less dramatic Doppler changes, but still carry increased stillbirth risk 1.