Retrograde menstruation — the backflow of menstrual blood through the fallopian tubes into the pelvic cavity — has long been implicated in endometriosis development, yet clinicians have lacked a reliable, non-invasive method to measure it in living patients. A practical imaging approach that correlates with menstrual blood loss volume could fundamentally change how gynecologists screen for and monitor endometriosis risk, bypassing the need for diagnostic laparoscopy in early assessment.
This prospective observational cohort study enrolled 102 women at a tertiary infertility center and performed standardized transvaginal ultrasound (TVUS) at two defined cycle timepoints: days 2–3 (menstrual phase) and days 7–10 (mid-to-late proliferative phase). Free pelvic fluid (FPF) was detected in 71.6% of participants during menstruation, dropping significantly to 44.1% by the proliferative phase. Median FPF volume fell from 1.0 mL during menstruation to 0 mL by days 7–10. Critically, among women with measurable FPF, 73% showed a positive within-cycle change — a statistically significant departure from the 50% expected under a null hypothesis of no cycle-driven difference. FPF volumes also scaled meaningfully with menstrual blood loss: women with heavy flow showed greater than 1 mL of menstrual-phase FPF in 60.7% of cases, versus 43.9% in normal bleeders and 0% in scanty bleeders.
This work represents a meaningful methodological step toward objectifying retrograde menstruation, a phenomenon that, despite decades of research, has remained essentially unmeasurable in vivo. The ellipsoid formula applied here is a pragmatic adaptation of existing ultrasound technology — no specialized equipment is required. That said, important limitations temper enthusiasm: the single-center design, modest sample size of 102 participants, and observational structure preclude causal inference. The study cannot confirm that FPF detected by TVUS directly predicts endometriosis development, only that it correlates with menstrual blood loss. Future longitudinal studies tracking FPF across cycles and correlating it with endometriosis staging would substantially strengthen the clinical utility of this surrogate marker. For now, this is a promising proof-of-concept with incremental but genuine clinical relevance.