Retrospective cohort study on preparation regimens for frozen embryo transfer

Objective: To examine the pregnancy outcomes from frozen embryo transfer (FET) cycles using different endometrial preparation regimens, compared to ovulation induction with letrozole (letrozole OI). Design: Retrospective cohort study. Setting: Fertility centre in Sydney, Australia. Patient(s): 6060 frozen embryo transfer cycles. Interventions: Cycles were stratified into one of four ways to achieve endometrial preparation. These were either a natural, letrozole OI, OI with follicle stimulating hormone (FSH OI) or a programmed cycle. Main Outcome Measure(s): The primary outcome was live birth rate per embryo transfer (LBR). Secondary outcomes included clinical pregnancy and biochemical pregnancy rates, adverse events including miscarriage, ectopic pregnancy, stillbirth, neonatal death and multiple births. Ovarian stimulation parameters were also analysed including time taken to reach the luteal phase and the number of blood or urine tests required for monitoring of the cycle. Results: The LBR following letrozole OI cycles was higher when compared to natural cycles (OR 1.27 (1.07 – 1.49)) and programmed cycles (OR 2.36 (1.67 – 3.34)). There was no significant difference between letrozole OI and FSH OI LBR (OR 0.99 (0.76 – 1.28)). An improved LBR with letrozole OI compared to natural cycles was maintained when only women with a normal length cycle were considered (OR 1.44 (1.10 – 1.89)). There was a significant reduction in miscarriage rates when letrozole OI was compared to programmed cycles (OR 0.46 (0.26 – 0.83)). Conclusion(s): The use of letrozole OI for endometrial preparation in an FET cycle may be associated with higher LBR and lower miscarriage rate, compared to using a programmed cycle.