Fig. 2

Dose–response relationship between paternal smoking and preterm birth. A paternal smoking; B preconception paternal smoking. The black curves represent adjusted odds ratios for paternal smoking based on restricted cubic splines with knots at the 5th, 35th, 65th, and 95th percentiles of smoking amount. Grey lines show the 95% confidence intervals of odds ratio for restricted cubic splines of paternal smoking (Parts of the lines were clipped for better comparison). Covariates used in the analysis included region of province, maternal age at last menstrual period, paternal age at last menstrual period, higher education, living area, Han ethnic, body mass index, alcohol drinking, parental passive smoking, thyroids stimulating hormones, diabetes, hypertension, parity, and history of adverse pregnancy outcomes. Red dashed lines demonstrate the reference level