The microbiology of amniotic fluid sludge: a systematic review and meta-analysis

Amniotic fluid sludge (AFS) is a sonographic finding associated with intra-amniotic infection and spontaneous preterm birth. The microbial composition of AFS remains poorly characterized. This systematic review and meta-analysis aimed to consolidate existing data on microbial prevalence in AFS and identify consistent microbial patterns.

A systematic search of MEDLINE (PubMed), Dimensions, and OpenAlex was conducted from inception to February 11, 2026. Studies reporting the prevalence of microorganisms in AFS (minimum 10 cases) from pregnant women with AFS diagnosed via transvaginal ultrasound were included. Two reviewers independently performed screening, data extraction, and risk-of-bias assessment, with a third reviewer resolving discrepancies. Random-effects meta-analysis of proportions was used to pool overall and pathogen-specific prevalence.

Four studies comprising 85 women met inclusion criteria. The pooled prevalence of any microorganism was 35% (95%CI 22-48%), with moderate heterogeneity (I² = 36.8%). Ureaplasma spp. were the most frequently detected pathogens (pooled prevalence 17%, 95%CI 8-27%). Sensitivity analysis revealed that sampling technique was a source of heterogeneity in this dataset; excluding the single study using transvaginal amniocentesis reduced the pooled prevalence to 28% (95%CI 17-41%) and eliminated statistical heterogeneity (I² = 0.0%). Transvaginally collected samples exhibited significantly greater microbial diversity (median 2 vs. 1 microorganism/sample, p = 0.005) and contained vaginal commensals, suggesting possible contamination.

In transabdominal cohorts, the pooled prevalence of microorganisms (any) in amniotic fluid sludge was 28% (95%CI 17-41%), with Ureaplasma spp. emerging as the predominant pathogen when an infectious agent was identified. Transvaginal amniocentesis resulted in greater microbial diversity and detection of vaginal commensals, consistent with contamination. Our findings suggest that transabdominal sampling should be the preferred methodological approach for future studies of AFS microbiology.