Joint effect of the indoor air pollution index and ambient PM₁ on fetal undergrowth. (a) Indoor air pollution exposure was defined as No if the indoor air pollution index was 0; otherwise, it was defined as Yes. (b) Adjusted for maternal age, ethnicity, employment status, maternal educational level, parity, pre-pregnancy overweight or obesity, gestational hypertension, gestational diabetes mellitus, alcohol drinking, physical activity, dietary calorie and ambient temperature. (c) A Wald test was performed on the interaction terms to evaluate the interaction between the indoor air pollution index and ambient PM₁. (d) Relative excess risk of fetal undergrowth due to interaction using the method of variance estimates recovery (MOVER) method. AC, abdominal circumference; HC, head circumference; FL, femur length; EFW, estimated fetal weight; RERI, relative excess risk due to interaction; PM₁, particulate matter with diameter of <1 μm (PM₁).