Plot of SF (μg/L) concentrations with median sTfR1 (mg/L) by trimester. First trimester (A), second trimester (B), third trimester (C), and unknown trimester (D); note that the behavior of the median becomes erratic with the small numbers of participants at the highest SF levels). In addition, related RCS regression with 5 knots is shown. First trimester (E), second trimester (F), third trimester (G), and unknown trimester (H); vertical line indicates the plateau point; shaded areas inside the dashed lines are 95% CIs. Data from a healthy sample of pregnant women (15-49 years) who participated in the NHANES from 1999 to 2010 and 2015 to 2018.