P-120 Seasonal effects at the time of oocyte collection on the success of frozen embryo transfers
Authors: Leathersich S , Hart R , Roche C , Walls M 
Published: June 2023

Study question

Does season and duration of daylight hours at the time of oocyte collection impact on live birth rates following frozen embryo transfer?

Summary answer

Frozen embryo transfer following oocyte collection in summer had 30% increased odds of live birth compared to oocytes collected in autumn.

What is known already

Season and conditions at the time of fresh or frozen embryo transfer do not appear to impact live birth rates. Recent data from the northwest USA suggest increased live birth rates following frozen embryo transfer from oocytes collected in summer, independent of the season at the time of transfer.

Study design, size, duration

Retrospective cohort study of all frozen embryo transfers at a single centre in Western Australia from 2013 - 2021 for which oocyte collection also occurred between these dates. This study included 3,659 frozen embryo transfers with embryos generated from 2,155 IVF cycles in 1,835 patients. Data were analysed by season, temperature and recorded sunshine hours at the time of oocyte collection and embryo transfer.

Participants/materials, setting, methods

All transfers during the study period were included; demographics, IVF cycle characteristics, embryological data and clinical outcomes were collected from the clinic database. Weather data were recorded by the Australian Bureau of Meteorology. Statistical adjustment was performed for factors known to affect live birth rates, and we corrected for multiple cycles in the same patient and conditions at embryo transfer.

Main results and the role of chance

Compared to frozen embryo transfer (FET) with oocyte retrieval dates in autumn, FET with oocyte retrieval dates in summer had 30% increased odds of live birth (OR: 1.30, 95% CI: 1.04-1.62). Temperature at the time of oocyte retrieval did not affect LBR; there was a 28% increase in odds of live birth when the sunshine hours were in the highest tertile compared to the lowest on the day of oocyte retrieval (OR: 1.28, 95% CI: 1.06-1.53). These findings remained consistent when adjusted for season or sunshine hours on the day of FET respectively.

Odds of livebirth were decreased by 18% when the minimum temperature on the day of transfer was high compared with low (OR: 0.82, 95% CI: 0.69−0.99).

The duration of bright sunshine on the day of oocyte retrieval appeared to drive the seasonal variations, whereas ambient temperature was not associated with clinical outcomes.

Limitations, reasons for caution

This was a retrospective study, however study populations across seasons, temperature tertiles and sunshine tertiles were similar. We did not analyse subgroups of low prognosis patients. We also did not analyse environmental pollutants in this study, nor did we assess impacts on sperm quality. These may all affect the results.

Wider implications of the findings

Optimal conditions for livebirth appear to be associated with summer and increased sunshine hours on the day of oocyte retrieval. Consideration could be given to timing oocyte retrieval to optimise outcomes. The mechanisms remain to be clarified, though may involve melatonin and its impact on oocyte quality.

Trial registration number

2022/ET000980