Fig. 1: Maternal serum metal concentrations and risk of preterm birth.

A Forest plots of OR and 95% CI for the relationship of serum metal concentrations in the first and second trimesters with preterm birth were estimated using the mixed effect model. Model was adjusted for baseline age, BMI, secondhand smoking status, education, spouse education, income levels, marriage age, and number of previous pregnancies. B Forest plots of OR and 95% CI for the relationship of serum metal concentrations in the first trimester with preterm birth were estimated using the logistic model. Model was adjusted for baseline age, BMI, secondhand smoking status, education, spouse education, income levels, marriage age, and number of previous pregnancies. C Forest plots of OR and 95% CI for the relationship of serum metal concentrations in the second trimester with preterm birth were estimated using the logistic model. Model was adjusted for baseline age, BMI, secondhand smoking status, education, spouse education, income levels, marriage age, and number of previous pregnancies.