Analyzing risks of adverse pregnancy outcomes.

Approaches for analyzing the risks of adverse pregnancy outcomes have been the source of much debate and many publications. Much of the problem, in our view, is the conflation of time at risk with gestational age at birth (or birth weight, a proxy for gestational age). We consider the causal questions underlying such analyses with the help of a generic directed acyclic graph. We discuss competing risks and populations at risk in the context of appropriate numerators and denominators, respectively. We summarize 3 different approaches to quantifying risks with respect to gestational age, each of which addresses a distinct etiological or prognostic question (i.e., cumulative risk, prospective risk, or instantaneous risk (hazard)) and suggest the appropriate denominators for each. We show how the gestational age-specific risk of perinatal death (PND) can be decomposed as the product of the gestational age-specific risk of birth and the risk of PND conditional on birth at a given gestational age. Finally, we demonstrate how failure to consider the first of these 2 risks leads to selection bias. This selection bias creates the well-known crossover paradox, thus obviating the need to posit common causes of early birth and PND other than the study exposure.