Urinary cotinine levels versus self-reported smoking status for assessing placental abruption risk: the Japan Environment and Children's study.

Introduction: Placental abruption is a serious complication of pregnancy, and smoking is a well-known risk factor thereof. However, evaluations of the effects of smoking mostly rely on self-reported questionnaires, resulting in incorrect representation of smoke exposure owing to incorrect reporting and missing data on passive smoking. Cotinine is a major nicotine metabolite and a sensitive biomarker in smokers. However, to date, there have been no reports on the relationship between urinary cotinine levels and onset of placental abruption. Hence, this study aimed to compare smoking status assessed using self-reports and urinary cotinine levels with placental abruption.

Material and methods: We analyzed data from the largest nationwide Japanese birth cohort study conducted between January 2011 and March 2014. Women with multiple pregnancies and insufficient data were excluded, leaving 82,637 women eligible for the study. Participants were classified as nonsmokers (<0.31 µg/gCr), passive smokers (0.31-36.7 µg/gCr), or active smokers (≥36.8 µg/gCr) based on comparable cutoffs for urinary cotinine levels, as reported previously. Participants were also classified as never-smokers, quitting before pregnancy, quitting after pregnancy, or current smokers according to a self-reported questionnaire. Logistic regression was used to calculate the adjusted odds ratios (aORs) with 95% confidence intervals (CIs) for the association of urinary cotinine levels and self-reported smoking status with placental abruption.

Results: Among 82,637 participants (mean age: 31.3 years), 346 (0.42%) had placental abruption. Based on urinary cotinine-assessed smoking status, active smokers had a higher incidence of placental abruption (aOR, 1.70; 95% Cl, 1.20-2.40) than nonsmokers. In contrast, based on self-reports, there was no significant difference between current smokers and never-smokers after adjusting for established confounding factors (aOR, 1.54; 95% CI, 0.98-2.36).

Conclusions: Elevated urinary cotinine levels during pregnancy were significantly associated with an increased risk of placental abruption, whereas self-reported smoking status was not. Urinary cotinine levels are a more accurate and clinically reliable biomarker to assess smoking-related risks during pregnancy than self-reported smoking status. Future research should explore the relationship between urinary cotinine levels and other obstetric complications to validate its utility as a comprehensive pregnancy biomarker.