Abstract
OBJECTIVE: To investigate the association between smoking and infertility. DESIGN: Prospective study. SETTING: Nationwide cohort. PATIENTS: 28,606 women and 27,096 men with questionnaire and genotype information from the Norwegian Mother, Father, and Child Cohort Study. INTERVENTION: Self-reported information on smoking (having ever smoked [both sexes], age at initiation [women only], cessation [women only], and cigarettes/week in current smokers [both sexes]) was gathered. Genetically predetermined levels or likelihood of presenting these traits were estimated for Mendelian randomization. MAIN OUTCOME MEASURE: Infertility (time-to-pregnancy ≥12 months). RESULTS: Having ever smoked was unrelated to infertility in women or men. Higher smoking intensity in women was associated with greater infertility odds (+1 standard deviation [SD, 48 cigarettes/week]: odds ratio [OR](crude,) 1.19; 95% confidence interval [CI] 1.11-1.28; OR(adjusted) 1.12; 95% CI, 1.03-1.21), also after adjusting for the partner's tobacco use. Later smoking initiation (+1 SD [3.2 years]: OR(crude,) 0.94; 95% CI, 0.88-0.99; OR(adjusted) 0.89; 95% CI, 0.84-0.95) and smoking cessation (vs. not quitting: OR(crude,) 0.83; 95% CI, 0.75-0.91; OR(adjusted,) 0.83; 95% CI, 0.75-0.93) were linked to decreased infertility in women. Nevertheless, Mendelian randomization results were not directionally consistent for smoking intensity and cessation and were estimated imprecisely in the 2-sample approach. In men, greater smoking intensity was not robustly associated with infertility in multivariable regression and Mendelian randomization. CONCLUSIONS: We did not find robust evidence of an effect of smoking on infertility. This may be due to a true lack of effect, weak genetic instruments, or other kinds of confounding.