Association between adenomyosis of the inner myometrial layer and chronic endometritis: a retrospective cohort study

Is diffuse adenomyosis involving the inner myometrial layer independently associated with chronic endometritis in women with infertility, and women who have experienced recurrent pregnancy loss (RPL) or repeated implantation failure (RIF)?

Retrospective cohort study conducted at two tertiary referral centres (January 2023‒ June 2025). Women aged 18-45 years undergoing evaluation for infertility, RPL or RIF underwent transvaginal 2D/3D-ultrasound, hysteroscopy and endometrial biopsy were included. Internal diffuse adenomyosis was defined by MUSA criteria, restricted to diffuse disease of the inner myometrial layer. Chronic endometritis was diagnosed by CD138 immunohistochemistry. Multivariable logistic regression adjusted for predefined confounders, with sensitivity analyses to assess robustness.

A total of 332 women were included; 48 (14.5%) had internal diffuse adenomyosis, and 284 (85.5%) had none. Chronic endometritis was significantly more prevalent in women with internal diffuse adenomyosis than in those without (41.7% [20/48] versus 13.7% [39/284] P < 0.001). After adjusting for age, uterine fibroids, prior uterine surgery and parity, internal diffuse adenomyosis remained independently associated with chronic endometritis (OR 4.33, 95% CI 2.16 to 8.70, P < 0.001). Sensitivity analyses yielded consistent results. Subgroup analyses showed a significantly higher prevalence of chronic endometritis in women with internal diffuse adenomyosis compared with those without across infertility (55.6% [15/27] versus 16.6% [28/169], P < 0.001), and RPL (26.7% [4/15] versus 8.6% [6/70], P = 0.048).

Internal diffuse adenomyosis is associated with chronic endometritis in women with infertility, and women who have experienced RPL and RIF outcomes. These findings support the concept that endometrial inflammation may represent a relevant biological component of adenomyosis-associated reproductive dysfunction and warrant further investigation into its pathophysiologic and clinical implications.