Stochastic mediation contrasts in epidemiologic research: interpregnancy interval and the educational disparity in preterm delivery.

Low maternal education is consistently associated with increased risk of preterm delivery (PTD). The interpregnancy interval (IPI), defined as the time between the date of a previous birth and the conception date of the index pregnancy, may mediate this relationship. We estimated controlled direct effects to assess whether hypothetical interventions designed to increase IPIs would reduce the educational disparity in PTD. We introduce a technique for estimating controlled direct effects under interventions that set only some persons in the population to a specific mediator value. We used data from 847,618 singleton livebirths occurring in Quebec, Canada, between 1989 and 2010. Compared with mothers with some university education (≥14 years), mothers with less than high school (<11 years), high school (11 years), and some college (12-13 years) had excess PTD risks of 2.6% (95% confidence interval (CI): 2.4, 2.8), 1.5% (95% CI: 1.4, 1.7), and 1.0% (95% CI: 0.9, 1.1), respectively. Risk differences under an intervention corresponding to the Healthy People 2020 objective of reducing the number of mothers with IPIs less than 18 months by 3% were no different from those for the total relationship. Our results suggest that interventions designed to increase the length of short IPIs will yield no important change in the PTD disparity by maternal educational level.