Perinatal epidemiology often aims to evaluate exposures on infant outcomes. When the exposure affects the composition of people who give birth to live infants (e.g., by affecting fertility, behavior, or birth outcomes), this "live birth process" mediates the exposure effect on infant outcomes. Causal estimands previously proposed for this setting include the total exposure effect on composite birth and infant outcomes, controlled direct effects (e.g., enforcing birth), and principal stratum direct effects. Using perinatal HIV transmission in the SEARCH Study as a motivating example, we present two alternative causal estimands: 1) conditional total effects; and 2) conditional stochastic direct effects, formulated under a hypothetical intervention to draw mediator values from some distribution (possibly conditional on covariates). The proposed conditional total effect includes impacts of an intervention that operate by changing the types of people who have a live birth and the timing of births. The proposed conditional stochastic direct effects isolate the effect of an exposure on infant outcomes excluding any impacts through this live birth process. In SEARCH, this approach quantifies the impact of a universal testing and treatment intervention on infant HIV-free survival absent any effect of the intervention on the live birth process, within a clearly defined target population of women of reproductive age with HIV at study baseline. Our approach has implications for the evaluation of intervention effects in perinatal epidemiology broadly, and whenever causal effects within a subgroup are of interest and exposure affects membership in the subgroup.

翻译：围产期流行病学常旨在评估暴露对新生儿结局的影响。当暴露改变活产婴儿母亲的群体构成（例如通过影响生育能力、行为或分娩结局）时，这一"活产过程"中介了暴露对新生儿结局的效应。针对这一场景，先前提出的因果估计量包括：复合分娩与新生儿结局的总体暴露效应、受控直接效应（如强制分娩）以及主分层直接效应。以SEARCH研究中围产期HIV传播为例，我们提出两种替代性因果估计量：1）条件总效应；2）条件随机直接效应，后者通过假设干预将中介变量值从某个分布（可能条件于协变量）抽取而构建。所提出的条件总效应包含通过改变活产人群构成及分娩时间影响结果的干预效应。所提出的条件随机直接效应则排除通过活产过程产生的影响，单独量化暴露对新生儿结局的效应。在SEARCH研究中，该方法在明确定义的基线HIV阳性育龄女性目标人群中，量化了全民检测与治疗干预对新生儿无HIV生存率的效应，且不包含干预对活产过程的影响。本方法对围产期流行病学中的干预效应评估具有广泛意义，并适用于任何关注亚组因果效应且暴露会影响亚组隶属度的场景。