Assessment of the Fertile Window in Subfertile Patients Using the P-Type Crystallization Biomarker in Liquid Endocervical Biopsy: A Prospective Study

Background: The assessment of the fertility window can be approached from the point of view of the analysis of biochemical and hormonal changes related to the ovarian physiology of the hypothalamic-pituitary-gonadal axis. These changes at the target organ level determine the biophysical characteristics of cervical secretion. The present study examined the Peak (P)-type crystallization pattern of cervical secretions in relation with to the clinical assessment of the fertile window. Methods: A prospective study was conducted in 45 subfertile patients undergoing sterility evaluation following Natural Procreative Technology and Restorative Reproductive Medicine approaches. 18 of the 45 subfertile patients met the inclusion criteria and completed a sterility work-up, along with a multidisciplinary assessment of the fertile window. In this selected sample, the P-type crystallization pattern was analyzed during the fertile window by performing a liquid endocervical biopsy. Finally, a randomly selected subgroup of 6 patients was assessed for pregnancy outcomes. Results: The fertile window was assessed in all participants. 18 digital captures (40%) were true positives, with P-type crystallization observed and a sensitivity of 100%. 27 captures (60%) were true negatives, with 100% specificity. Zero false-negatives or false-positives were observed. Live-birth pregnancy was achieved in 83% (5/6) of patients with positive P-type crystallization results in the randomly selected subgroup. The most fertile-window days were identified in all cases between three days before the estimated day of ovulation (EDO) until peak-day P (-3 EDO until P-Day). P-type crystallization, characterized by maximal branching and a tricolor pattern in a hexagonal configuration, was observed in 100% of the study group. Conclusions: This study supports the integration of cervical secretion elasticity and transparency assessment with crystallization analysis, within an objective reference framework. The triad of (a) high-quality cervical secretion, (b) accurate fertile window diagnosis, and (c) the hexagonal P-type crystallization pattern may collectively contribute to predicting positive pregnancy outcomes. Clinical Trial Registration: The study has been registered on https://www.isrctn.com/ (registration number: ISRCTN18705969; registration link: https://www.isrctn.com/ISRCTN18705969).