Systemic complement C3 reduction in infertile women with endometrial dysbiosis: preliminary observations

Context Endometrial dysbiosis has been linked to infertility, but its relationship with systemic immune activity remains unclear. Aims To examine whether endometrial microbiota phenotype is associated with serum complement profiles in infertile women. Methods We enrolled 105 infertile women and classified endometrial microbiota by 16S rRNA gene sequencing as Lactobacillus-dominant microbiota (LDM) or non-LDM (NLDM). Complement data were available in 27 women: after excluding cases with missing C-reactive protein (CRP; n = 2) or CRP above the 95th percentile (n = 2), 23 women were analyzed for C3/C4 and 19 for CH50. Group comparisons used Welch’s t-test and Fisher’s exact test. Key results NLDM was associated with lower serum C3 than LDM (101.5 ± 12.4 vs 121.0 ± 18.0 mg/dL; P = 0.006). C4 and CH50 did not differ significantly between groups. Conclusions Endometrial dysbiosis was associated with selective C3 reduction in this infertile cohort, consistent with mild complement consumption. Implications Serum C3, interpreted with microbiota profiling, may provide a practical adjunct marker of inflammation-related infertility in routine care. Larger prospective studies are needed to validate these preliminary findings.