Structural equation model for the effect of arsenic on birth weight. The diagram illustrates the estimated path from arsenic exposure to birth weight. Variables are represented by rectangles. Each path is represented by a line with an arrow or vertical bar at one end. Arrow denotes positive association and vertical bar denotes negative association. The calculated coefficient estimate and 95% CI for each path are: in utero arsenic exposure (U-As) and placental AQP9 expression (coefficient estimate: 0.25; 95% CI: 0.05 – 0.45); placental AQP9 expression and ENPP2 expression (coefficient estimate: -0.13; 95% CI: -0.22 – -0.02); placental ENPP2 expression and infant birth weight (coefficient estimate: 0.28; 95% CI: 0.09 – 0.47). The indirect path from U-As to infant birth weight has a coefficient estimate of −0.009 and a 95% CI of −0.032 to −0.001 (n = 133 and 10000 replications for bootstrapping).