SDC1 and chronic endometritis: <h4>Study question</h4>What is the relationship between constitutive CD138 expression in the endometrium and the reproductive tract microbiota composition?<h4>Summary answer</h4>The presence of CD138+ cells in endometrial stroma is cycle-dependent and associated with impaired luteal phase endometrial timing but not altered vaginal or endometrial microbial composition.<h4>What is known already</h4>CD138-diagnosed chronic endometritis (CE) is associated with adverse reproductive outcomes including recurrent pregnancy loss (RPL) in uncontrolled studies.