ORs of extremely preterm, very preterm and moderately preterm births after adjustments for characteristics and risk factors.

*(The contribution of each factor was measured by the percentage reduction in the odds ratio of socioeconomic status compared to Model 2 and 3 by using formula (OR Model 2/3– OR Model x)/(OR Model 2/3–1). Model 1 = Adjusted by socioeconomic status. Model 2 = Adjusted by socioeconomic status+age and parity. Model 3 = Adjusted by Model 2+ smoking. Model 4 = Adjusted by Model 3+ placental abruption+placenta previa+major congenital anomaly+anemia+stillbirth+SGA+ fetal sex. Model 5 = Adjusted by Model 3+ amniocentesis+chorionic villus biopsy+IVF.