**Question:** The influence of maternal smoking and LBW incidence is studied. Detailed smoking history is taken at first antenatal visit and smoking history and bih weight were studied later. The type of study is -

A. Case-control study
B. Cross-sectional study
C. Prospective cohort study
D. Randomized controlled trial

**Correct Answer:** C. Prospective cohort study

**Core Concept:** A prospective cohort study is a observational study design used to investigate the relationship between a risk factor (in this case, maternal smoking) and an outcome (low birth weight). It involves following a group of individuals (in this case, pregnant women) over time and collecting data on the risk factor and outcome.

**Why the Correct Answer is Right:** In this scenario, a prospective cohort study is the appropriate design to investigate the association between maternal smoking and low birth weight (LBW) incidence. The study involves collecting detailed smoking history at the first antenatal visit and monitoring both smoking status and bih weight (birth weight) over time. This allows for the examination of the relationship between smoking and LBW incidence in pregnant women.

**Why Each Wrong Option is Incorrect:**

A. Case-control study: This study design compares cases (individuals with the outcome) to controls (individuals without the outcome) to identify risk factors. In this case, it would not be suitable as it cannot establish a temporal relationship between smoking and LBW.

B. Cross-sectional study: This study design assesses the prevalence of risk factors and outcomes at a single point in time, making it unsuitable to investigate the relationship between smoking and LBW incidence over time.

D. Randomized controlled trial: This is an experimental study design involving the assignment of participants to different interventions (e.g., smoking cessation vs. no intervention). Since the study aims to understand the relationship between smoking and LBW, a randomized controlled trial is not appropriate.

**Clinical Pearl:** Prospective cohort studies are essential for understanding the temporal relationship between risk factors and outcomes. They provide valuable information for public health interventions and preventive measures, such as counseling pregnant women to quit smoking to reduce LBW risk.