Investigating the Relationship between Salivary Estriol to Progesterone Ratio and Onset of Labor: A Feasibility Study

Background: Salivary hormone levels of estriol (E3) and progesterone (P) have been studied as potential biomarkers for preterm and labor. Prior studies have reported a rise in the E3 to P ratio (E3:P) in the weeks leading up to spontaneous labor as well as an association between elevated E3 and preterm labor. This study sought to clarify if there was a daily rise in E3:P in term labor that could be captured in salivary samples and potentially indicate the onset of labor. Methods: This was a single-center prospective pilot study aimed at evaluating E3:P in term pregnant females from 38 weeks until day of delivery. Secondary outcomes included E3, P, and E3:P variability and labor outcomes (spontaneous vaginal delivery, cesarean section, or induction). Participants self-collected daily salivary samples starting at 38 weeks gestational age leading up to delivery. A Wilcoxon signed-rank test was used to evaluate E3:P, estriol, and progesterone change prior to delivery. A Mann-Whitney U was utilized to compare median E3:P, estriol, and progesterone between the induced and spontaneous groups. Results: We found no significant change in E3:P prior to labor onset (p=0.81) We also did not find a significant rise in estriol or progesterone alone prior to labor onset between subjects (p=0.58 and p=0.23). E3:P and estriol were significantly lower in those that were induced versus those who went into spontaneous labor (p<0.001 and P<0.001). Conclusions: Although this study had limited power, higher E3:P ratios were significantly associated with spontaneous labor compared to induced labor, suggesting E3:P may serve as a clinical decision making tool for induction planning in term pregnancy.