FIGURE 1.

Causal diagram representing associations between maternal smoking, abnormal placentation, early pregnancy loss, and preeclampsia. Collider bias, denoted by the dashed line connecting maternal smoking and preeclampsia, arises in observational studies which implicitly restrict to pregnancies that survive past the point of early loss. Some simulation scenarios represent settings that assume null associations between smoking and early loss (edge α), smoking and abnormal placentation (edge β), or abnormal placentation and early loss (edge γ), such that arrows representing those associations would be absent from the figure. All simulations were based on the assumption that early loss precludes the occurrence of preeclampsia (edge δ).