Dados do Trabalho

Title

Association between meteorological season at the time of embryo transfer and pregnancy outcomes in freeze-all cycles: lessons from seasonal reproductive mammals

Objective

Extensive research has consistently shown that most mammals are seasonal breeders, and as day length increases, gonadotropin secretion increases, to anticipate reproduction in the spring and summer months. There is intriguing evidence that there is a seasonal variance in human reproduction, but the data are inconclusive. Therefore, the goal for the present study was to evaluate the effect the meteorological season at the time of embryo transfer on clinical outcomes of freeze-all cycles.

Methods

This historical cohort study, performed in a private university-affiliated IVF centre between February 2018 and December 2022, included 4952 frozen-thawed embryos transferred to 3725 cycles in a single clinic. Both vitrification and the warming procedures were performed using the Cryotop method. Embryos survival and clinical outcomes were analyzed by the season, at the time of embryo transfer, using generalized linear models adjusted for potential confounders.

Results

The mean maternal age was 37.0 ± 4.7 years. Embryos were cryopreserved for a mean of 8.1 ± 17.3 months. Autumn was designated as the reference season for all analyses. No significant differences were observed in embryo survival rates when the seasons were compared, however, lowest implantation and pregnancy rates were observed when embryo transfers occurred in autumn (44.6%), followed by winter (45.8%, OR: 1.049, CI: 0.876 – 1.257, p=0.603) and summer (49.0%, OR: 1.196, CI: 1.010 – 1.415, p=0.038), while spring embryo transfers had a 30% increased odds of pregnancy (51.2%, OR: 1.302, CI: 1.100 – 1.541, p=0.002). An increased miscarriage rate was observed when embryo transfers occurred in autumn (13.9%), followed by winter (11.4%, OR: 0.510, CI: 0.323 – 0.805, p=0.004) and summer (10.4%, OR: 0.461, CI: 0.292 – 0.727, p=0.001), while spring embryo transfers had a 63% decreased odds of miscarriage (5.6%, OR: 0.369, CI: 0.230 – 0.592, p<0.001).

Conclusion

In seasonal breeding mammals, melatonin, synthesized in the pineal gland, leads increased release of gonadotropins during the transition period (between anestrus and estrus onset, usually after winter and before summer). Melatonin rise has been proved to favor embryo implantation and protect offspring by increasing progesterone levels. In the present study, optimal conditions for embryo implantation appeared to be associated with spring and summer. In contrast, frozen embryo transfers performed in autumn and winter are associated with reduced ongoing pregnancy rates. Our evidence raises the question of how human reproductive potential may be influenced by climatic variations and whether this information may be useful in assisted reproduction cycles.

Keywords

cryopreservation; embryo development; embryo transfer; environmental effects; infertility; meteorological seasons.