Circadian serum progesterone variations on the day of frozen embryo transfer in artificially prepared cycles

Research question: What is the intra-day variation of serum progesterone related to vaginal progesterone administration on the day of frozen embryo transfer (FET) in an artificial cycle? Design: A prospective cohort study was conducted including 22 patients undergoing a single blastocyst artificial cycle (AC)–FET from August to December 2022. Endometrial preparation was achieved by administering oestradiol valerate (2 mg three times daily) and consecutively micronized vaginal progesterone (MVP; 400 mg twice daily). A blastocyst FET was performed on the 6th day of MVP administration. Serum progesterone concentrations were measured on the day of transfer at 08:00, 12:00, 16:00 and 20:00 hours. The first and last blood samples were collected just before MVP was administered. Results: The mean age and body mass index of the study population were 33.95 ± 3.98 years and 23.10 ± 1.95 kg/m2. The mean P-values at 08:00, 12:00, 16:00 and 20:00 hours were 11.72 ± 4.99, 13.59 ± 6.33, 10.23 ± 3.81 and 9.28 ± 3.09 ng/ml, respectively. A significant decline, of 2.41 ng/ml (95% confidence interval 0.81–4.00), was found between the first and last progesterone measurements. Conclusion: A statistically significant intra-day variation of serum progesterone concentrations on the day of FET in artificially prepared cycles was observed. This highlights the importance of a standardized procedure for the timing of progesterone measurement on the day of AC–FET. Of note, the study results are applicable only to women using MVP for luteal phase support; therefore it is necessary to confirm its validity in comparison with the different existing administration routes of progesterone.