• Shiro Tanaka, Maki Shinzawa, Hironobu Tokumasu, Kahori Seto, Sachiko Tanaka, and Koji Kawakami.
• Department of Pharmacoepidemiology, Graduate School of Medicine and Public Health, Kyoto University, Yoshida-Konoe-cho, Sakyo-ku, Kyoto 606-8501, Japan.
• BMJ. 2015 Jan 1;351:h5397.

Study QuestionDoes maternal smoking during pregnancy and exposure of infants to tobacco smoke at age 4 months increase the risk of caries in deciduous teeth?MethodsPopulation based retrospective cohort study of 76 920 children born between 2004 and 2010 in Kobe City, Japan who received municipal health check-ups at birth, 4, 9, and 18 months, and 3 years and had information on household smoking status at age 4 months and records of dental examinations at age 18 months and 3 years. Smoking during pregnancy and exposure of infants to secondhand smoke at age 4 months was assessed by standardised parent reported questionnaires. The main outcome measure was the incidence of caries in deciduous teeth, defined as at least one decayed, missing, or filled tooth assessed by qualified dentists without radiographs. Cox regression was used to estimate hazard ratios of exposure to secondhand smoke compared with having no smoker in the family after propensity score adjustment for clinical and lifestyle characteristics.Study Answer And LimitationsPrevalence of household smoking among the 76 920 children was 55.3% (n=42 525), and 6.8% (n=5268) had evidence of exposure to tobacco smoke. A total of 12 729 incidents of dental caries were observed and most were decayed teeth (3 year follow-up rate 91.9%). The risk of caries at age 3 years was 14.0% (no smoker in family), 20.0% (smoking in household but without evidence of exposure to tobacco smoke), and 27.6% (exposure to tobacco smoke). The propensity score adjusted hazard ratios of the two exposure groups compared with having no smoker in the family were 1.46 (95% confidence interval 1.40 to 1.52) and 2.14 (1.99 to 2.29), respectively. The propensity score adjusted hazard ratio between maternal smoking during pregnancy and having no smoker in the family was 1.10 (0.97 to 1.25).What This Study AddsExposure to tobacco smoke at 4 months of age was associated with an approximately twofold increased risk of caries, and the risk of caries was also increased among those exposed to household smoking, by 1.5-fold, whereas the effect of maternal smoking during pregnancy was not statistically significant.Funding, Competing Interests, Data SharingThis study was supported by a grant in aid for scientific research 26860415. The authors have no competing interests or additional data to share.© Tanaka et al 2015.

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