Strengths and limitations
The strengths of our study include the large cohort size that allows classification of LM population according to different causes, and specific populations confined to women who had only one LM after the first embryo transfer to eliminate other confounding factors as possible. In addition, there were also several limitations in our study. First, this study was limited by the retrospective design from a single center, including missing data, potential recording bias. Second, the sample sizes were relatively limited in some subgroups, such as feLM and trLM group, which may have a bearing on the statistical accuracy. Last, the causes of LM were only categorized into embryonic, cervical, traumatic and unexplained factors, more specific categorizations with clearer etiology are still needed in future studies.