Interaction between maternal passive smoking during pregnancy and joint genotype of CYP1A1 BsrD I and GSTs on risk of spontaneous PTD.

a.ORa adjusted for family income, maternal age, education level, prepregnancy BMI and CYP1A1 Msp I genotype.b.GSTs “null” if both GSTM1 and GSTT1 “null”, GSTs “present” if either GSTM1 or GSTT1 “present”.c.% for distribution within the case and control groups respectively.d.The variable of passive smoking+CYP1A1 BsrD I + GSTs with eight levels was set as dummy independent variable, and the first level as reference category.