P-644 The effect of endometrial preparation protocol on pregnancy outcomes and obstetric complications in frozen-thawed embryo transfer cycles

Abstract Study question Is there an association between endometrial preparation protocol in ART cycles with frozen-thawed embryo transfers and rates of pregnancy and obstetric complications? Summary answer Natural cycle protocols had higher ongoing pregnancy rates than hormonal protocols, despite older maternal age. Hormonal protocols required more uterine cavity assessments (P = 0.03) What is known already The literature lacks consensus on the superiority of any protocol regarding pregnancy and live birth rates. Programmed cycles are linked to higher rates of pregnancy-induced hypertension (PIH), pre-eclampsia (PET), postpartum hemorrhage (PPH), preterm delivery (PTD), placental abruption (PA), large for gestational age (LGA), low birthweight, early pregnancy loss, and preterm labor. Study design, size, duration Retrospective cohort study of patients undergoing frozen-thawed embryo transfers at an IVF unit in a university medical center between 2011-2021. Exclusion criteria: egg donation cycles, loss to follow-up before pregnancy testing, and endometrial preparation using letrozole or gonadotropin cycles. Participants/materials, setting, methods Women undergoing frozen-thawed embryo transfer with: Programmed cycle (PC): Continuous estrogen followed by progesterone. Natural cycle (NC) or modified natural cycle (mNC): Monitored by ultrasound and lab tests until LH surge and progesterone rise. mNC included 250 mcg recombinant hCG (Ovitrelle). Progesterone supplements were administered in both NC and mNC. Embryo transfer was performed by a senior physician under ultrasound guidance using a soft catheter. Data was sourced from electronic medical records of IVF unit and labor rooms. Main results and the role of chance The study included 2,793 treatment cycles (2,350 PC, 443 NC/mNC). Ongoing pregnancy and live birth rates were higher in natural/modified natural groups compared to hormonal (16.2% vs. 11.7%, P = 0.008), despite older maternal age. No significant differences in mean endometrial thickness or number of embryos transferred. Hormonal cycles had more pregnancies but also more chemical pregnancies and early miscarriages. No significant differences in delivery modes or obstetric complications. Hormonal group had more primary PPH cases and greater need for uterine cavity and birth canal assessment (P = 0.03). Limitations, reasons for caution The retrospective nature of the study is a major limitation. Selection of endometrial preparation protocol was not detailed, potentially influenced by infertility background, previous IVF treatment history, and obstetric history. Prospective randomized studies are needed. Wider implications of the findings Natural and modified natural cycles may be preferable for achieving ongoing pregnancies and live births, despite the need for more careful monitoring in hormonal cycles. Further prospective studies are required to confirm these results and explore underlying mechanisms. Trial registration number Yes