The aim of this study was to assess the obstetric outcomes of letrozole induced frozen-thawed embryo transfers cycles This single arm prospective cohort study, conducted between 2020 and 2022 at a private assisted reproductive technologies (ART) center, focused on infertile patients undergoing FET. The study included FET cycles with single, good quality blastocysts on day 5 (Gardner Class A or B), while excluding patients with irregular menses, oligomenorrhea, or polycystic ovarian syndrome. Ultrasound assessments were performed on the second or third day of the menstrual cycle to evaluate any endometrial and ovarian pathology. Letrozole (5 mg) was administered for five days. Subsequently, regular ultrasound checks were conducted to monitor endometrial pattern and follicle growth. Human chorionic gonadotropin (hCG) trigger was planned when the follicle diameter reached 17-18 mm. Vaginal micronized progesterone (3x200 mg) was prescribed as routine luteal phase support, starting on the second day of hCG trigger. Embryo transfer took place on the 7th day following hCG trigger. Additionally, subcutaneous progesterone (25 mg) was added to the luteal phase support twice a week. A positive beta-human chorionic gonadotropin (b-hCG) test was defined as a level greater than 5 IU/dl on the 9th day of FET. Clinical pregnancy was determined by the presence of a visible gestational sac on ultrasound, and live birth was characterized by delivery after the 24th week of pregnancy. Demographic parameters, cycle outcomes, and obstetric outcomes (delivery week, birth weight, preterm birth, pre-eclampsia) were recorded. A total of 122 FET cycle were enrolled in. mean woman age was 30 + 4.6 yrs. The infertility etiologies were unexplained (25%), male factor (30%), tubal factor (10%), anovulation (20%) and diminished ovarian reserve (15%). the median progesterone level was 25.9 ng/dl on ET day. Positive b-HCG rate was 52.5% (64/122). Clinical pregnancy rate was 49.2% (60/122). Miscarriage rate was 8.2% (10/122) and live birth rate was 44% (60/122). Preterm birth was occurred in 6 patients (11%). There were two pregnancies with intrauterine growth restriction (3.7%) and pre-eclampsia was developed in two patients (3.7%). (Table 1) The serum progesterone levels were compared between patients with or without positive pregnancy test. And patients with negative pregnancy test had slightly higher progesterone levels. (27.9 vs. 24.4, p=0.3) (Table 2,3) In FET cycles, endometrial preparation using letrozole appears to be a safe alternative when compared to hormone replacement cycles, as indicated by existing literature. However, it is important to consider that a high serum progesterone level (27.9 ng/dl) may have a negative impact on implantation due to a potential ceiling effect. [ABSTRACT FROM AUTHOR]