An epidemiologist in Mumbai is investigating whether maternal anemia during pregnancy is associated with preterm birth. She plans to recruit 300 pregnant women without anemia and 300 pregnant women with anemia, then follow both groups until delivery to record preterm birth outcomes. Which study design best describes this investigation?

Explanation

## Identifying Prospective Cohort Study Design ### Key Structural Features of This Study **Key Point:** The study recruits participants based on **exposure status** (anemia vs. no anemia) at baseline and follows them **forward in time** to observe the outcome (preterm birth). This is the defining characteristic of a prospective cohort study. **High-Yield:** Prospective cohort studies are the investigation of choice when: - Participants are classified by **exposure status at entry** (not by disease status) - The outcome has **not yet occurred** at baseline - You follow participants **forward in time** (prospectively) - You can calculate **relative risk directly** - The exposure is common and the disease is not rare ### Cohort vs. Case-Control: Fundamental Distinction | Dimension | Prospective Cohort | Case-Control | |-----------|-------------------|---------------| | **Starting point** | Exposure (exposed vs. unexposed) | Outcome (cases vs. controls) | | **Direction of inquiry** | Forward in time (exposure → disease) | Backward in time (disease → exposure) | | **Outcome at baseline** | Not yet occurred | Already present | | **Risk measure** | Relative Risk (RR) | Odds Ratio (OR) | | **Efficiency** | Poor for rare diseases | Excellent for rare diseases | | **Time required** | Long (months to years) | Short (weeks to months) | | **Best for** | Common exposures, common outcomes | Rare diseases, long latency periods | **Clinical Pearl:** In maternal health research, prospective cohort studies are standard because pregnancy is a defined time window, outcomes are relatively common (preterm birth ~5–10% in India), and you can follow women from early pregnancy to delivery. **Mnemonic: EXPOSE First** — Exposure-based recruitment, Prospective follow-up, Outcome measured later = Cohort study ### Why This is NOT Case-Control ~~Case-control would start with preterm birth cases and controls, then ask retrospectively about anemia history.~~ Instead, this study starts with anemia exposure status and follows women forward to see who develops preterm birth — that's cohort design.