P-511 Prolonging the time of vaginal progesterone supplementation does not improve pregnancy outcomes after day 6 blastocyst transfer
Authors: Blazquez A , Popovic M , MIguel-Escalada I , Colomé C , Rodriguez F , Rodriguez A 
Published: June 2023

Study question

Does the administration of vaginal progesterone for 6 days improve pregnancy outcomes when transferring day 6 blastocysts, as opposed to 5 days of progesterone treatment?

Summary answer

We observed similar pregnancy outcomes after 5 and 6 days of vaginal progesterone supplementation following day 6 blastocyst transfer in oocyte recipient cycles.

What is known already

Progesterone administration promotes the final phase of endometrial preparation for embryo transfer. However, the optimal duration of progesterone transformation still remains unknown. The implantation potential of blastocysts formed at different timepoints is also contentious, as blastocysts on day 6 have been linked to poorer clinical outcomes compared to their day 5 counterparts. It is unclear whether this effect stems from an intrinsic impaired implantation capacity of slow-developing blastocysts, or a displaced window of implantation (WOI). It has been hypothesized that 6 days of progesterone administration may improve clinical outcomes after frozen day 6 blastocyst transfer.

Study design, size, duration

Retrospective cohort study of oocyte recipients undergoing single day 6 blastocyst transfer, between January 2020 and December 2022. This study compared two groups: day 6 blastocysts transferred after 5 days of progesterone administration (n = 284) and day 6 embryo transfers performed after 6 days of progesterone administration (n = 69).

Participants/materials, setting, methods

Our study included oocyte recipients who underwent a single embryo transfer of a day 6 blastocyst and hormonal replacement treatment for endometrial preparation with vaginal external progesterone administration (800mg/24h) for either 5 or 6 days prior to embryo transfer. Outcomes were analyzed using unpaired t-test or Fisher’s test and logistic regression. Multivariate models were adjusted for oral or transdermal estrogen administration and endometrial thickness on the day of embryo transfer. P-values &lt;0.05 were considered significant.

Main results and the role of chance

Mean maternal age was 43.5 years, while patients had a mean BMI of 25.1. Mean days of estrogen therapy prior to embryo transfer were 22.4, with 75.3% of patients using transdermal estrogen administration, and achieving a mean endometrial thickness of 9.6 mm on the day of transfer. Mean progesterone levels on the day of transfer were 11.9 ng/ml. Thirty-seven percent of the day 6 blastocysts were of high quality. None of the demographic or cycle characteristics were significantly different between the study groups. When considering outcomes of day 6 blastocyst transfers, we found no significant differences in biochemical (26.33% vs 20.13%, p = 0.343), clinical (22.06% vs 17.46%, p = 0.49) and ongoing (14.64% vs 9.52%, p = 0.41) pregnancy rates between 5 and 6 days of progesterone administration. Our adjusted analysis further corroborated these results.

Limitations, reasons for caution

The retrospective nature of the study and low number of transfers performed after 6 days of progesterone treatment warrant careful interpretation. This study only evaluated endometrial preparation by vaginal progesterone administration in oocyte recipients and cannot be extrapolated to other routes of progesterone administration or other patient populations.

Wider implications of the findings

Poorer reproductive outcomes associated with the transfer of day 6 blastocysts appear to be related to a reduced implantation capacity of slow-developing embryos rather than a displaced WOI due to rising progesterone. WOI appears wider than assumed, as prolonged progesterone transformation times had no effect on pregnancy outcomes.

Trial registration number

Not applicable