Free pelvic fluid detection and quantification using transvaginal ultrasound: a surrogate marker for retrograde menstruation

Research question: Can transvaginal ultrasonography (TVUS) reliably detect and quantify free pelvic fluid (FPF) as a surrogate marker of retrograde menstruation, and how do its volume and characteristics vary across menstrual phases, characteristics and gynaecological conditions? Design: Prospective, observational, single-centre cohort study at a tertiary infertility centre. Women underwent two standardized TVUS examinations within the same natural menstrual cycle: on day 2–3 (menstrual phase) and day 7–10 (medium–late proliferative phase). FPF volume was measured using the ellipsoid formula, and within-cycle changes (ΔVRM) were calculated. The primary outcome was the presence and extent of FPF and within-cycle FPF changes, and the secondary outcome was menstrual-phase FPF changes across menstrual blood loss (MBL) categories. Results: In total, 102 women were included in this study. FPF was detected in 71.6% (95% CI 61.8–80.1%) on day 2–3 and in 44.1% (95% CI 34.3–54.3%) on day 7–10 (P < 0.001). Among women with measurable FPF (n = 73), ΔVRM was positive in 73% (95% CI 61.4–82.7%; P < 0.001), significantly different from the 50% expected under no within-cycle difference. Median FPF volume declined from 1.0 ml (IQR 0.3–3.2) ml in the menstrual phase to 0 ml (IQR 0–0.6 ml) in the proliferative phase (P < 0.001). Menstrual FPF >1 ml was observed in 0% (0/4), 43.9% (18/41) and 60.7% (17/28) of women with scanty, normal and heavy MBL, respectively (P = 0.03). Conclusion: Menstrual FPF was detected in over two-thirds of women, with within-cycle volumetric changes, wide variability between women, and increasing magnitude with heavier MBL. These findings suggest that, using FPF as a surrogate marker, retrograde menstruation may be identified and quantified using a simple non-invasive TVUS-based method. This work offers a potential methodological framework for studying menstrual physiology.