Objective: Age at natural menopause (ANM) is usually defined as the age at the last menstrual bleeding followed by the absence of menses for 12 consecutive months. Although many studies have suggested an association between smoking and early age at natural menopause, evidence remains conflicting because some studies reported inconsistent or contrasting results. To resolve this ambiguity and to quantitatively evaluate the effect of smoking on ANM, we conducted a meta-analysis of the available data about smoking and ANM.
Methods: After extensive searching of public literature databases, a total of 11 studies were selected for this meta-analysis. Among them, the phenotype of the participants in five studies (dichotomous studies) was classified as early or late ANM, and odds ratio (OR) was used to evaluate the effect of smoking on early ANM. For the other six studies (continuous studies), mean and SD were provided for smoking and nonsmoking samples, and weighted mean difference (WMD) was used as the effect size.
Results: We found that smoking was significantly associated with early ANM in both dichotomous and continuous studies. The pooled effect was OR = 0.74 (95% CI, 0.60-0.91, P < 0.01) in the dichotomous studies. For the continuous studies, the pooled effect estimated by WMD was -1.12 (95% CI, -1.80 to -0.44, P = 0.04). After adjustment of the original data for heterogeneity, the pooled results changed only a little: OR = 0.67 (95% CI, 0.61-0.73, P < 0.01) for dichotomous studies and WMD = -0.90 (95% CI, -1.58 to -0.21, P = 0.01) for the continuous studies.
Conclusions: The results of our study suggest that smoking is a significant independent factor for early ANM.