Maternal dietary-diversity counselling in primary-care antenatal clinics: a target-trial emulation estimating birthweight effects under imperfect programme uptake

Antenatal dietary-diversity counselling is widely promoted in primary care, yet its birthweight effects remain uncertain because programme uptake is incomplete and clinic-level clustering inflates the variance of any naive comparison. the present analysis specifies a target-trial emulation for a before-after-control-impact design and characterises its operating characteristics under a declared variance structure: between-cluster standard deviation 6.0, residual standard deviation 4.5, and intraclass correlation 0.64. Power was estimated across 2000 replications per design cell for cluster counts of 8, 16, 24 and 32 and effects of 3, 5 and 8 percentage points.52 clusters are required to detect a 5 percentage point effect at 80% power, that the minimum detectable effect at 80% power falls steeply as cluster count rises from 8 to 32, and that the distribution of estimated power is wide at small cluster counts. The contribution is a design-level account of what a target-trial emulation can and cannot identify when uptake is imperfect, and a demonstration that cluster count, not effect size alone, governs the credibility of birthweight inferences in this setting.