Strengths and limitations
This study has several advantages: (i) The strengths of this study include the comprehensive search strategies, a double review process, and stringent selection criteria. The study also used a standardized tool to evaluate the quality of the literature. (ii) In our systematic review, we included only studies that were conducted in the community-based population so that our results could be more accurate and representative. (iii) We strictly limited the definition of infertility in our study to ensure the comparability of the combined prevalence. The standardized definitions of infertility subtypes and an adequate subgroup analysis reduced heterogeneity.
Several limitations of this study should also be recognized. First, although we unified the definitions of the prevalence of infertility and its subtypes before pooling the prevalence estimates, substantial heterogeneity was detected. Besides, even for the prevalence of infertility, for which the contributing data points successfully covered all the 6 WHO regions, the prevalence estimation at the regional level was not optimal given that more than half of the included studies were concentrated in only 2 regions (Asian Region and European Region), and the small number of studies in some countries which may not represent the true estimate in that countries.