O-026 A new mNC protocol that allows a 7-day window for FET planning

Abstract Study question Is triggering necessary at 17 mm follicle diameter in a modified natural cycle (mNC), or does it allow flexible planning? Summary answer Embryo transfer can be scheduled when follicles measure 13 to 20 if endometrium is ready without impacting clinical outcome. What known already Current practice shifting, moving frozen embryo transfers (FET) from artificial cycles cycles, which may complicate planning. The standard mean trigger mNC has classically been stablished mm, mimicking the size needed obtain mature oocytes. Interestingly, research on different sizes limited. A previous study initiated progesterone based only endometrial ultrasonographic characteristics and presence of dominant least 12 showed good results ongoing pregnancy rates. However, had small sample rhCG was not administered. design, size, duration This multicenter, retrospective, observational 3,087 single blastocyst mNCs carried out 2,764 patients our centers January 2020 September 2022. Participants/materials, setting, methods Selection criteria were following: day 5/6 (minimum quality 3BB attending Gardner classification), regular menstrual (26-35 days), normal uterine cavity assessed by ultrasound, serum <1.5 ng/mL thickness ≥7 administration rhCG, absence fluid cavity. Triggering done with dose 250 µg sc micronized 200mg bid started two days later, then SET performed. Main role chance Follicle time stratified into three groups (13.0–15.9 mm; 16.0–18.9 ≥19.0 mm). No differences seen regarding age, body mass index (BMI) years infertility, however, there egg donation (39.5%; 27.9%; 27.4% respectively; p = 0.02) use Preimplantational Genetic Testing for Aneuploidies (PGT-A) (19.4%; 34.01%; 37.3%; < 0.01). We found no rate (64.5%; 60.2%; 57.4%; 0.19), (60.5%; 52.8%; 50.6%; 0.10), implantation (62.10%; 52.9%; 51.0%; 0.05) miscarriage (15.0%; 22.2%; 25.0%; 0.11), but (OPR) (54.9%; 46.8%; 43.1%; 0.02). those after adjusting PGT-A donation: OPR vs 13.0–15.9 (aOR 2.37; 95% CI: 0.73–7.60; 0.15) > 19mm 0.75; 0.54–1.05; 0.10). Finally, each millimeter (80,0%; CI 29.9—99.0%) 22 (54.6%; 39.0—69.3%). Limitations, reasons caution 15 19 accounted 84.7% included this study, leaves minority cases “non conventional” sizes. need confirmed future prospective studies. Wider implications findings Our show that could administrated 22mm. Considering follicular growth 1-1.5mm per day, approach flexibility five seven days, facilitating planning FET practice. Trial registration number Not applicable