Objective Pilot study to identify relationships between body temperature and levels of peripheral P4 blood progesterone, and examine if these differ according to body temperature cycle pattern. Design Prospective, observational study. Setting IVIRMA IVF clinics in Madrid, Mallorca and Malaga, Spain. Population 62 data points from 18 patients undergoing hormone assisted embryo-implantation cycles that volunteered to use OvuSense, an intra-vaginal body temperature monitor. Main Outcome Measures OvuSense Raw and Smooth Temperature (°C), P4 (ng/ml). Results A graphical analysis showed an apparent relationship between P4 levels and Temperature taken on P4 blood draw day. A multilevel regression analysis using MLwiN 3.10 ¹ software investigated this relationship, allowing between-patient variation to be accounted for and estimated. This established a strong linear relationship between LnP4 and ST, and cross correlation was carried out which identified the optimum predictor of levels of LnP4 was ST measured on the day prior to blood sampling. Further graphical analyses showed an apparent lower luteal level of P4 for cycles flagged as atypical by OvuSense, and for negative outcomes, except on embryo transfer day. Conclusions The results provide extremely strong evidence (Z = 15.6, p < 0.0001, 2 sided) of a linear relationship between LnP4 and Smooth Temperature (ST) measured the day before blood sampling. This suggests that ST could provide a less invasive, continuous, and more practical method of assessing P4 response. Further investigation is required to establish the value for improving outcomes.