Researchers in Delhi are investigating whether maternal anemia during pregnancy is associated with low birth weight (LBW) in newborns. They enroll 300 mothers who delivered LBW infants (< 2500 g) and 300 mothers who delivered normal birth weight infants (≥ 2500 g) from three tertiary hospitals. Both groups are interviewed about their hemoglobin levels during pregnancy and other maternal factors. The researchers calculate an Odds Ratio of 2.8 (95% CI: 1.9–4.1) for the association between maternal anemia and LBW. A colleague argues that a prospective cohort study would have been superior. Which statement best justifies why the case-control design was actually more practical for this research question?

Explanation

## Case-Control vs Cohort: Practical Efficiency in Perinatal Epidemiology ### Why Case-Control Excels for LBW Studies **Key Point:** Although LBW is a *relatively common* outcome in India (15–20% of deliveries), a case-control design is still more efficient than a prospective cohort because: 1. **Cases are readily available** — LBW infants are already identified at delivery in hospital records 2. **Retrospective exposure assessment** — maternal hemoglobin and anemia status can be extracted from antenatal records or maternal recall 3. **No loss to follow-up** — the outcome (birth weight) is already known; no need to follow women through pregnancy 4. **Rapid recruitment** — cases and controls can be enrolled immediately postpartum ### Cohort Study Burden (Why It Would Be Less Practical) A prospective cohort would require: - Enrolling a large cohort of pregnant women at baseline (first trimester) - Measuring hemoglobin and other exposures prospectively - Following all participants through 9 months of pregnancy - Waiting for delivery to ascertain the outcome (birth weight) - Managing attrition, loss to follow-up, and migration - **Time to results:** 12–18 months minimum **Clinical Pearl:** Even though LBW is relatively common (not "rare"), case-control remains efficient because: - The outcome is already determined (you don't have to wait for pregnancy to conclude) - Exposure data are already documented in medical records - You can match cases and controls on confounders (maternal age, parity, socioeconomic status) to increase efficiency ### Comparison Table | Feature | Case-Control | Cohort | |---------|--------------|--------| | **Time to completion** | 3–6 months | 12–18 months | | **Cost** | Lower (no follow-up) | Higher (long-term follow-up) | | **Attrition risk** | Minimal | Moderate to high | | **Suitable for common outcomes** | Yes, if cases are accessible | Yes, but less efficient | | **Odds Ratio** | Directly calculated | Derived from RR | **High-Yield:** The **rarity of the outcome** is not the only criterion for choosing case-control. **Accessibility of cases** and **feasibility of exposure assessment** are equally important. **Mnemonic: FAST = Case-Control for This Scenario** **F** — Feasible: cases already identified at hospitals **A** — Accessible: antenatal records available **S** — Swift: no 9-month follow-up needed **T** — Time-efficient: results in months, not years [cite:Park 26e Ch 8]