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STUDY QUESTION Is summer associated with a higher live birth rate after fresh IVF/ICSI? SUMMARY ANSWER There was no support for a higher live birth rate after fresh IVF/ICSI when treatment was performed during the summer season. WHAT IS KNOWN ALREADY Seasonal variations in human natural conception and birth rates are well described. It has been hypothesized that serum vitamin D, levels of which are associated with sun exposure, may have a role in human natural conception rates. However, the association between seasons and IVF outcomes has not yet been clarified and conflicting reports have been published. Furthermore, it has been suggested that women with normal vitamin D levels have a better pregnancy outcome after ART compared to those with vitamin D insufficiency. STUDY DESIGN, SIZE, DURATION A nationwide, register-based cohort study including all first-time fresh IVF/ICSI treatments (n = 52 788) leading to oocyte retrieval in Sweden between 2009 and 2018 was carried out. PARTICIPANTS/MATERIALS, SETTING, METHODS All first-time fresh IVF/ICSI cycles leading to oocyte retrieval were identified in the National Quality Registry of Assisted Reproduction. Data collected included patient characteristics as well as information about the treatment cycle and pregnancy outcome. The patients were divided into season subgroups, (summer, autumn, winter and spring) based on the date of oocyte retrieval. The primary outcome was live birth rate, which was defined as the number of live births per oocyte retrieval and embryo transfer (ET). Other outcomes included clinical pregnancy per ET and miscarriage per clinical pregnancy. Logistic regression with multiple imputation was performed to evaluate whether there was an association between season and IVF/ICSI outcomes, with summer as reference. Adjustments were made for woman's age, year of treatment, BMI, total FSH/hMG dose, type of treatment, fertilization type, embryonic stage at ET and number of embryos transferred. MAIN RESULTS AND THE ROLE OF CHANCE Live birth rate per oocyte retrieval ranged between 24% and 26% among seasons. A significantly higher live birth rate was seen for spring compared with summer, 26% versus 24%, respectively (adjusted odds ratio (OR) 1.08, 95% CI 1.02–1.16, P  =   0.02). No significant association was seen when winter and autumn were compared with summer. Live birth rate per ET ranged between 29% and 31% among seasons. A significantly higher live birth rate was seen for spring and autumn compared with summer, at 31% and 31%, respectively versus 29% (adjusted OR 1.08, 95% CI 1.01–1.16, P  =   0.04 and adjusted OR 1.09, 95% CI 1.01–1.16, P  =   0.02), respectively. No significant association was seen when winter was compared with summer. Clinical pregnancy rate varied between 36% and 38% and miscarriage rate between 16% and 18%, with no significant seasonal associations. LIMITATIONS, REASONS FOR CAUTION Possible limitations are the retrospective design of the study and unmeasured confounders. Another limitation is that a generalized estimating equation (GEE) model was not used. The use of a GEE model would have made it possible to include all started fresh IVF/ICSI cycles since it allows for correction for any dependence between cycles within women. WIDER IMPLICATIONS OF THE FINDINGS The results of this large registry study give no support for the hypothesis that IVF/ICSI treatments performed during summer season, with the highest degree of sunlight and vitamin D synthesis, is associated with higher pregnancy and live birth rates. In fact, our results showed significantly lower live birth rates during summer compared with spring and autumn. However, the magnitude of this difference was small and unlikely of clinical value. We suggest that season should not be taken into consideration when planning and performing fresh IVF/ICSI treatments. STUDY FUNDING/COMPETING INTEREST(S) Financial support was received through the Swedish state under the agreement between the Swedish government and the county councils, the ALF-agreement (ALFGBG-70 940) and grants from the Hjalmar Svensson's Research Foundation (HJSV2021019 and HJSV2021037). None of the authors declare any conflict of interest. TRIAL REGISTRATION NUMBER N/A. [ABSTRACT FROM AUTHOR]

Background: Uterine transplantation could serve as a tool in studies of the physiology of implantation/pregnancy, and is also a possible future treatment for patients with absolute uterine infertility. Here, the first live-born offspring in any uterine transplantation model is reported., Methods: A syngeneic mouse model with a uterus transplanted, by end-to-side aorta/vena cava vascular anastomoses, alongside the native uterus was used. The cervix was attached to a cutaneous stoma. Pregnancy rate and offspring (birth weight, growth and fertility) was evaluated after blastocyst transfer to the native and the grafted uterus of transplanted mice and to controls., Results: Pregnancy rates were comparable in the grafted uterus (8/12 animals became pregnant) and the native uterus (9/12 pregnant) of transplanted animals and controls (8/13 pregnant). In a separate set of animals, the native uterus was removed at transplantation to exclude influences from the native uterus on the pregnancy potential of the graft; two of four animals became pregnant after blastocyst transfer. The weights/lengths of fetuses (gestational day 18) and gestational lengths were similar in all groups. Offspring were delivered and the growth trajectories (up to 8 weeks) of offspring delivered from grafted or native uteri of transplanted mice were similar as compared with controls, and all were fertile. The second-generation offspring from transplanted animals were all fertile with normal birth weights., Conclusions: These observations document the capacity of a transplanted uterus to harbour pregnancies to term, and reveal that offspring from a transplanted uterus develop to normal fertile adults.