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63 A COHORT STUDY ON FACTORS IMPAIRING SEMEN QUALITY IN TRANSGENDER WOMEN 4 concentration. 101,102 In this way, ejaculation frequency and abstinence time will most likely not have an impact TMSC, as TMSC takes the absolute value of three semen parameters into consideration simultaneously. Clinical implications The impaired pre-freeze semen quality in our cohort has implications for future use of cryopreserved semen, since freezing and thawing further decreases sperm motility. We observed a large proportion of samples with low post-thawTMSC which has consequences for many transgender women, since 40.7% of our cohort reported to be solely attracted to women and 35.3% were attracted to both women and men. Therefore, the future use of cryopreserved semen with a female partner is a realistic scenario and implicates that the majority will need an invasive and burdensome IVF or ICSI treatment to start a family. The results of our study provide a possibility to optimize fertility counseling by encouraging transgender women to minimize wearing tight undergarment and tucking before pursuing semen cryopreservation. However, since only a minority of transgender women reported to perform these habits, these results do not explain the overall decreased semen quality in transgender women and part of the etiology remains unknown. Research implications For future research it would be worthwhile to further assess the impact of GAHT on spermatogenesis and especially its restoration after cessation of this treatment, in a longitudinal manner. Hereby, we might be able to establish an optimal strategy and explore the feasibility of natural conception for transgender women with a female partner and a desire for genetically related offspring. Strengths and limitations The major strength of this study is the prospective design which enabled us to collect detailed information on habitual behavior more typically observed in transgender women, including the frequency and duration of tucking, as well as information about all other relevant factors (i.e. demographics, lifestyle and medical history). As a result, we were able to accurately assess the influence of ejaculation frequency, wearing tight undergarment, and tucking on semen quality, while correcting for potential confounders. A limitation of this study is that some results are based on self-reported behavior and lifestyle which can lead to bias because of its subjective nature. However, no viable alternative is available. Furthermore, since this is the first study reporting on ejaculation frequency and tucking frequency in transgender women, it was not possible to use reference values from previous reports. Therefore, the stratification of these factors was based on clinical relevance.