P-481 Additional sub-cutaneous progesterone: a putative strategy to counteract poor clinical outcomes after vitrified-warmed euploid blastocyst transfer?

Abstract Study question Is there an association between additional subcutaneous progesterone (a-sP) during luteal phase support for vitrified-warmed euploid blastocyst transfers and the clinical outcomes? Summary answer a-sP was mainly administered to worse-prognosis patients:positive-pregnancy-test (PPT), but also biochemical-pregnancy-loss (BPL) rates were lower than control, thereby involvingsimilar live-birth-rate (LBR) per transfer. What is known already Frozen-embryo-transfer (FET) implementation is increasing worldwide, and luteal-phase-support is a hot-topic. Serum/uterine progesterone levels are key in eliciting immunological tolerance and uterine quiescence, while supporting the processes underlying embryo implantation. Yet, a consensus is missing on progesterone administration (formulation, route, dosage, and duration). Endometrial biopsy studies showed increased progesteroneconcentration after vaginal administration, while sP has excellent pharmacokinetics with faster peak serum concentration.Combining both administration routes might compensate cases ofpoor vaginal absorption and contribute to reaching adequate systemic/uterine levels. This synergy might be beneficial especially in RIF-patients, subject to lower LBRs also in the context of euploid ET. Study design, size, duration Retrospective analysis of 775 vitrified-warmed euploid blastocysttransfers conducted January2020-March2021. Three of 12 gynecologists administrated a-sP as luteal phase support mostly to patients with a worse reproductive history (i.e., longer duration of infertility and/or higher number of previous failed IVF). We assessed the clinical outcomes (PPT, BPL, miscarriage and LBR) in the group of patients who used a-SP (N = 128) versus the control(N = 647), adjusting for confounders in logistic regression analyses. Participants/materials, setting, methods Only euploid non-mosaic blastocysts were transferred. All patients had normal uterine cavities and thyroid function. Endometrial preparation was performed with either hormone-replacement-therapy (estradiol valerate 6mg/die plus vaginal micronized progesterone 800mg/die) or modified-natural-cycle (hCG administration plus vaginal micronized progesterone 400mg/die). In the a-sP group, the supplementation was started 3 days before FET. In case of pregnancy, the therapy was continued until the 8thweek. Main results and the role of chance The patients in the control and a-sP groups were similar for oocyte age (37.6±2.7yr versu