Circadian serum progesterone variations on the day of frozen embryo transfer in a modified natural cycle protocol
Authors: Loreti S , Roelens C , Aktoz F , Niero M , De Munck N , Tournaye H , Mackens S , Blockeel C 
Published: May 2024

Abstract

STUDY QUESTION
Is there a circadian variation of serum progesterone (P) on the day of frozen embryo transfer (FET) in a modified natural cycle (mNC)?

SUMMARY ANSWER
There is a statistically significant diurnal variation of serum P on the day of a FET in an mNC protocol.

WHAT IS KNOWN ALREADY
In recent years, the proportion of FET cycles has increased dramatically. To further optimize pregnancy outcomes after FET, recent studies have focused on serum luteal P levels in both natural and artificially prepared FET cycles. Despite the different cut-off values proposed to define low serum P in the NC, it is generally accepted that lower serum P values (&lt;10 ng/ml) around the day of FET are associated with negative reproductive outcomes. However, a single serum P measurement is not reliable given that P levels are prone to diurnal fluctuations and are impacted by patients’ characteristics.

STUDY DESIGN, SIZE, DURATION
A prospective cohort study was conducted in a single university-affiliated fertility center, including 22 patients performing a single blastocyst mNC-FET from August 2022 to August 2023. Serum P levels were measured on the day of transfer at 08:00h, 12:00h, 16:00h, and 20:00h. Differences between P levels were compared using the Wilcoxon signed-rank test. The sample size was calculated to detect a difference of 15% between the first and last P measurements with a 5% false-positive rate and a 95% CI.

PARTICIPANTS/MATERIALS, SETTING, METHODS
Patients with a normal BMI, between 18 and 40 years old, without uterine diseases were eligible. Patients utilizing donated oocytes were excluded. The mNC-FET protocol involved monitoring the normal ovarian cycle and triggering ovulation with an injection of 250 μg of choriogonadotropin alfa when a pre-ovulatory follicle (16–20 mm diameter) was visualized. The blastocyst was transferred seven days later. The patients were not supplemented with exogenous P at any time before the day of the FET.

MAIN RESULTS AND THE ROLE OF CHANCE
The mean age and BMI of the study population were 33.6 ± 3.8 years and 22.7 ± 1.8 kg/m2, respectively. Mean P values at 08:00h, 12:00h, 16:00h, and 20:00h were 14.6 ± 4.5, 14.7 ± 4.1, 12.9 ± 3.5, and 14.6 ± 4.3 ng/ml, respectively. The mean P levels at 16:00h were significantly lower compared to all other time points (P &lt; 0.05: P = 0.007 between P at 8:00h and 16:00h; P = 0.003 between P at 12:00h and 16:00h; P = 0.007 between P at 16:00h and 20:00h). No statistically significant difference was observed between P values at the other time points (P &gt; 0.05: P = 0.88 between P at 8:00h and 12:00h; P = 0.96 between P at 8:00h and 20:00h; P = 0.83 between P at 12:00h and 20:00h).

LIMITATIONS, REASONS FOR CAUTION
The study’s limitations include the small sample size that may cause a bias when the results are extrapolated to a larger subfertile population undergoing mNC-FET. Ideally, larger prospective trials including a more heterogeneous patient population would be necessary to validate our findings.

WIDER IMPLICATIONS OF THE FINDINGS
The current study demonstrates the existence of a diurnal fluctuation of serum P on the day of mNC-FET highlighting the importance of a standardized time point for its measurement. This is especially important for considering clinical actions, such as additional exogenous P supplementation, when encountering P values lower than 10 ng/ml on the day of FET.

STUDY FUNDING/COMPETING INTEREST(S)
No funding was obtained for the study. The authors have no conflicts of interest to declare regarding the content of the study.

TRIAL REGISTRATION NUMBER
NCT05511272.