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Study Question: To what extent is cigarette smoking associated with reduced fecundability?Summary Answer: Current female smokers, particularly those who had smoked ≥10 cigarettes/day for ≥10 years, had lower fecundability than never smokers, but current male smoking and passive smoking in either partner showed little association with reduced fecundability.What Is Known Already: Female smoking has been identified as a cause of infertility, yet there has been limited characterization of the dose and duration at which an effect is observed. Results for male active smoking and passive smoking in both partners are less consistent.Study Design, Size, Duration: We analyzed data from a North American internet-based preconception cohort study of 5473 female and 1411 male pregnancy planners, enrolled from 2013 to 2018. Participants had been attempting conception for ≤6 menstrual cycles at study entry.Participants/materials, Setting, Methods: We collected information on active and passive smoking history on baseline questionnaires. Pregnancy was reported on female bi-monthly follow-up questionnaires. We calculated fecundability ratios (FR) and 95% CI using proportional probabilities regression models, adjusted for demographic, behavioral, medical, reproductive and dietary variables.Main Results and the Role Of Chance: Female current regular smoking (FR = 0.90, 95% CI: 0.77, 1.07), current occasional smoking (FR = 0.88, 95% CI: 0.73, 1.06), and former smoking (FR = 0.89, 95% CI: 0.81, 0.98) were associated with small reductions in fecundability. Results were stronger among women who smoked ≥10 cigarettes/day for ≥10 years (FR = 0.77, 95% CI: 0.53, 1.10). Male current regular and former smoking, and current passive smoking in either partner were not meaningfully associated with reduced fecundability. In utero exposure to ≥10 cigarettes/day among females was associated with reduced fecundability (FR = 0.75, 95% CI: 0.52, 1.06).Limitations, Reasons For Caution: Numbers of cigarette smokers, particularly within categories of intensity and duration, were small. Under-reporting of smoking may have resulted in non-differential misclassification, and smokers were more likely to be lost to follow-up.Wider Implications Of the Findings: Given the consistency of our findings with results from previous studies and our observation of a dose-response relation in intensity of smoking, this study supports an association between female cigarette smoking and lower fecundability.Study Funding/competing Interest(s): This study was funded by the National Institute of Child Health and Human Development (R01-HD086742, R21-HD072326, R03-HD090315 and T32-HD052458). The authors declare no competing interests. [ABSTRACT FROM AUTHOR]