Low level of urinary cotinine in pregnant women also matters: variability, exposure characteristics, and association with oxidative stress markers.

China has relatively high indoor contamination of nicotine, according to recent publications. Therefore, nicotine exposure risks for sensitive populations such as pregnant women in China are of concern. The variability of its internal exposure levels across three trimesters among pregnant women is not well documented. Factors related with nicotine exposure across pregnancy and its associations with oxidative stress markers are also understudied. Based on a birth cohort, we measured concentrations of cotinine (a major metabolite of nicotine) and oxidative stress markers including 8-OHdG, 8-OHG, and HNE-MA in urine samples collected at three trimesters from 1,155 pregnant women enrolled between January 2014 and June 2017 in Wuhan, China. The variability of urinary cotinine across the trimesters, potential factors associated with it, as well as the relationships between urinary cotinine and oxidative stress markers were assessed in pregnant women with cotinine concentrations of < 50 ng/mL (the cutoff value to distinguish smokers and non-smokers). Urinary specific gravity adjusted median concentrations of cotinine (ng/mL) in the entire pregnancy, first, second, and third trimester were 3.04, 3.32, 3.36, and 2.50, respectively, which exhibited fair reliability (intraclass correlation coefficient: 0.47) across pregnancy. Most participants had an estimated daily intake of nicotine higher than the acceptable value (100 ng/kg-bw/day) recommended by the UK and the USA. Maternal age, education level, pre-pregnancy body mass index, and sampling seasons were related to urinary concentrations of cotinine. After adjusting for confounding factors, significant positive relationships (β; 95% confidence interval) were observed between urinary cotinine concentrations and 8-OHdG (0.28; 0.25, 0.30), 8-OHG (0.27; 0.25, 0.29), and HNE-MA (0.27; 0.21, 0.32), respectively (p < 0.01). These results lend insight into the major factors associated with nicotine exposure of pregnant women at environmentally relevant levels and its potential effect on oxidative stress with a large sample size, and warrant the necessity of reducing the exposure in sensitive populations.