Measuring dosage: a key factor when assessing the relationship between prenatal case management and birth outcomes.

To assess whether a measure of prenatal case management (PCM) dosage is more sensitive than a dichotomous PCM exposure measure when evaluating the effect of PCM on low birthweight (LBW) and preterm birth (PTB). We constructed a retrospective cohort study (N = 16,657) of Iowa Medicaid-insured women who had a singleton live birth from October 2005 to December 2006; 28 % of women received PCM. A PCM dosage measure was created to capture duration of enrollment, total time with a case manager, and intervention breadth. Propensity score (PS)-adjusted odds ratios (ORs), and 95 % confidence intervals (95 % CIs) were calculated to assess the risk of each outcome by PCM dosage and the dichotomous PCM exposure measure. PS-adjusted ORs of PTB were 0.88 (95 % CI 0.70-1.11), 0.58 (95 % CI 0.47-0.72), and 1.43 (95 % CI 1.23-1.67) for high, medium, and low PCM dosage, respectively. For LBW, the PS-adjusted ORs were 0.76 (95 % CI 0.57-1.00), 0.64 (95 % CI 0.50-0.82), and 1.36 (95 % CI 1.14-1.63), for high, medium, and low PCM dosage, respectively. The PCM dichotomous participation measure was not significantly associated with LBW (OR = 0.95, 95 % CI 0.82-1.09) or PTB (0.97, 95 % CI 0.87-1.10). The reference group in each analysis is No PCM. PCM was associated with a reduced risk of adverse pregnancy outcomes for Medicaid-insured women in Iowa. PCM dosage appeared to be a more sensitive measure than the dichotomous measure of PCM participation.