STUDY QUESTION Does the rate of miscarriage increase in the setting of adenomyosis independent of other known risk factors for miscarriage such as maternal age, BMI, embryo genetic status? SUMMARY ANSWER Adenomyosis and high BMI both significantly increase miscarriage risk independent of each other, maternal age and embryo health. This study is the first to suggest that ultra-long down regulation GnRH agonist treatment may reduce the rate of early pregnancy loss in adenomyosis patients. WHAT IS KNOWN ALREADY The presence of adenomyosis is known to be associated with lower rates of successful implantation and increased risk of early pregnancy loss. However, it is presently unclear whether this reproductive impairment is directly mediated by adenomyosis itself, or indirectly caused by adenomyosis association with known risk factors for miscarriage such as obesity and advancing maternal age/foetal aneuploidy. STUDY DESIGN, SIZE, DURATION A retrospective cohort study was undertaken in a private infertility (IVF) clinic examining the outcome for women ( n = 345) undergoing the transfer of a genetically screened frozen–thawed embryo between 2012 and 2015. PARTICIPANTS/MATERIALS, SETTING AND METHOD A total of 171 women who successfully conceived (positive serum βhCG) following the transfer of a single euploid good morphology frozen–thawed embryo were included in analysis after meeting the inclusion criteria. Only the first conception cycle for each patient was included in the study. Patients with known pre-existing medical risk factors for miscarriage (e.g. thrombophilia, poorly controlled diabetes, coeliac disease, SLE, uterine septum, chromosomal abnormalities) and those women undergoing treatment using donated oocytes and surrogacy were excluded. Patients were then classified as having adenomyosis or not based on a high-quality pelvic ultrasound or MRI. The direct and indirect effects of adenomyosis and BMI on overall miscarriage rate by 12 weeks gestation was then assessed using multivariate logistic regression and mediation analysis. Furthermore, the data were also analysed to elucidate the influence of GnRH ultra-long down-regulation therapy on miscarriage rates. MAIN RESULTS AND ROLE OF CHANCE Overall, the adjusted rate of miscarriage was higher in those patients with adenomyosis compared to those without (44.1 vs 15.3%, P < 0.0001), with most of these miscarriages occurring at the early biochemical stage. The rate of miscarriage was especially high in adenomyosis patients not receiving GnRH agonist pre-treatment (82.4%), compared to those patients who did receive GnRH pre-treatment (35.7%, P = 0.0089). LIMITATIONS, REASONS FOR CAUTION The study is mainly limited by its small sample size and retrospective design which carries in...

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