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STUDY QUESTION Does adjuvant letrozole in ovarian stimulation for IVF decrease the uterine peristalsis frequency (UPF) prior to fresh embryo transfer (ET)? SUMMARY ANSWER Adjuvant letrozole in ovarian stimulation for IVF does not reduce the UPF significantly prior to fresh ET. WHAT IS KNOWN ALREADY Throughout the cycle, uterine peristalsis aids spermatozoa transport to the fallopian tube and may affect implantation. At fresh ET, UPF is negatively correlated with implantation and clinical pregnancy rates and is believed to be modulated by oestradiol and progesterone. High levels of oestradiol, from multiple follicular development, in ovarian stimulation have been reported to increase UPF, whereas progesterone is considered to be an utero-relaxant. The influence of androgens is unclear. Co-treatment with letrozole during gonadotropin ovarian stimulation limits the supra-physiological oestradiol rise and may therefore reduce UPF prior to fresh ET. STUDY DESIGN, SIZE, DURATION This study was carried out on subjects participating in a single-centre double-blinded randomized controlled trial of the impact of letrozole on follicle development and endocrine profiles, and investigated the impact of adjuvant letrozole in ovarian stimulation for IVF on UPF prior to fresh ET and the correlations of UPF with endocrine markers. Between 2016 and 2017, 39 women expected to be normal responders were randomized to co-treatment with letrozole or placebo. Of these, 33 women completed this element of the study. The study was carried out according to the Helsinki Declaration and the ICH-Good-Clinical-Practice. PARTICIPANTS/MATERIALS, SETTING, METHODS Eligible women were randomized 1:1 to adjuvant treatment with letrozole 5 mg/day or placebo in an antagonist protocol using a fixed dose of recombinant (r) FSH 150 IU/day. Final maturation was triggered with hCG 6500 IU and luteal support with vaginal progesterone was administered from the day following oocyte aspiration. Less than 1 h pr

Context Supraphysiological sex steroid levels at the follicular-luteal phase transition are implicated as the primary cause of luteal insufficiency after ovarian stimulation (OS) for in vitro fertilization. Objective We aimed to determine the impact of suppressing estradiol levels during OS of multiple dominant follicles on the unsupported luteal phase and markers of endometrial maturation. Methods At 2 university hospitals, 25 eligible egg donors were randomized to undergo OS using exogenous gonadotropins with or without adjuvant letrozole 5 mg/day. Final oocyte maturation was triggered with a GnRH agonist. No luteal support was provided. The primary outcome was the duration of the luteal phase. Secondary outcomes were luteal phase hormone profiles and the endometrial transcriptomic signature 5 days after oocyte pick up (OPU + 5). Results The median (interquartile range [IQR]) luteal phase duration was 8.0 (6.8-11.5) days compared with 5.0 (5.0-6.8) days in the intervention and control group, respectively (P < 0.001). Estradiol levels were effectively suppressed in the letrozole group with a median of 0.86 (0.23-1.24) nmol/L at OPU compared to 2.82 (1.34-3.44) nmol/L in the control group. Median (IQR) progesterone levels at OPU + 5 were 67.05 (15.67-101.75) nmol/L in the letrozole group vs 2.27 (1.05-10.70) nmol/L in the control group (P < 0.001). In the letrozole group, 75% of participants revealed endometrial transcriptomic signatures interpreted as post-receptive. In the control group, 40% were post-receptive and 50% noninformative. Conclusion Suppressing estradiol levels in the follicular phase with adjuvant letrozole significantly reduces the disruption of the unsupported luteal phase after OS.

Study Question: Does letrozole co-treatment during ovarian stimulation with gonadotrophins for IVF reduce the proportion of women with premature progesterone levels above 1.5 ng/ml at the time of triggering final oocyte maturation? Summary Answer: The proportion of women with premature progesterone above 1.5 ng/ml was not significantly affected by letrozole co-treatment. WHAT IS KNOWN ALREADY: IVF creates multiple follicles with supraphysiological levels of sex steroids interrupting the endocrine milieu and affects the window of implantation. Letrozole is an effective aromatase inhibitor, normalizing serum oestradiol, thereby ameliorating some of the detrimental effects of IVF treatment. STUDY DESIGN, SIZE, DURATION: A randomized, double-blinded placebo-controlled trial investigated letrozole intervention during stimulation for IVF with FSH. The trial was conducted at four fertility clinics at University Hospitals in Denmark from August 2016 to November 2018. PARTICIPANTS/MATERIALS, SETTING, METHODS: A cohort of 129 women with expected normal ovarian reserve (anti-Müllerian hormone 8-32 nmol/l) completed an IVF cycle with fresh embryo transfer and received co-treatment with either 5 mg/day letrozole (n = 67) or placebo (n = 62), along with the FSH. Progesterone, oestradiol, FSH, LH and androgens were analysed in repeated serum samples collected from the start of the stimulation to the mid-luteal phase. In addition, the effect of letrozole on reproductive outcomes, total FSH consumption and adverse events were assessed. MAIN RESULTS AND THE ROLE OF CHANCE: The proportion of women with premature progesterone >1.5 ng/ml was similar (6% vs 0% (OR 0.0, 95% CI [0.0; 1.6], P = 0.12) in the letrozole versus placebo groups, respectively), whereas the proportion of women with mid-luteal progesterone >30 ng/ml was significantly increased in the letrozole group: (59% vs 31% (OR 3.3, 95% CI [1.4; 7.1], P = 0.005)). Letrozole versus placebo decreased oestradiol levels on

Letrozole reduces serum oestradiol by inhibiting the aromatase enzyme and has growing clinical indications in fertility. The available evidence of letrozole's role in ovarian stimulation for IVF and intracytoplasmic sperm injection (ICSI) and clinical outcomes was assessed. Medline, Cochrane, and ClinicalTrials.gov databases were systematically searched up until August 2021, including 31 studies (n = 16 randomized controlled trials [RCTs]; n = 15 observational studies). Live birth rate (LBR) in poor responders significantly increased by 7% (95% CI, 1% to 13%, P = 0.03) with letrozole co-treatment. Concomitantly, the gonadotrophin consumption was significantly reduced, without decreasing the number of retrieved oocytes. In normal responders, number of oocytes increased with 1.8 oocytes (95% CI 0.35 to 3.27, P = 0.01) with letrozole co-treatment. No significant effect on LBR, clinical pregnancy rate (CPR), or ovarian hyperstimulation syndrome rate was demonstrated. Only two studies reported on high responders and revealed no effect on LBR or CPR. Overall, the endometrium thickness was slightly affected, where as the, miscarriage rate and cancellation rate were unaffected by letrozole co-treatment. None of the included studies reported on neonatal outcomes. The quality of evidence was high or moderate in the RCTs and low in the observational studies. In conclusion, poor responders may benefit from co-treatment with letrozole during ovarian stimulation for IVF, whereas letrozole for normal and high responders requires further investigation with larger, high-quality studies.