Background. In many research settings, the intervention implied by the average causal effect of a time-varying exposure is impractical or unrealistic, and we might instead prefer a more realistic target estimand. Instead of requiring all individuals to be always exposed vs. unexposed, incremental effects quantify the impact of merely shifting each individual's probability of being exposed. Methods. We demonstrate estimation of incremental effects in the time-varying setting, using data from the Effects of Aspirin in Gestation and Reproduction trial, which assessed the effect of preconception low-dose aspirin on pregnancy outcomes. Compliance to aspirin or placebo was summarized weekly and was affected by time-varying confounders such as bleeding or nausea. We sought to estimate what the incidence of pregnancy by 26 weeks post-randomization would have been if we shifted each participant's probability of taking aspirin or placebo in each week by odds ratios (OR) between 0.30 and 3.00. Results. Under no intervention (OR=1), the incidence of pregnancy was 77% (95% CI: 74%, 80%). Decreasing women's probability of complying with aspirin had little estimated effect on pregnancy incidence. When we increased women's probability of taking aspirin, estimated incidence of pregnancy increased, from 83% (95% CI: 79%, 87%) for OR=2 to 89% (95% CI: 84%, 93%) for OR=3. We observed similar results when we shifted women's probability of complying with placebo. Conclusions. These results estimated that realistic interventions to increase women's probability of taking aspirin would have yielded little to no impact on the incidence of pregnancy, relative to similar interventions on placebo. Copyright © 2022 Wolters Kluwer Health, Inc. All rights reserved.
