The researchers also found a 28 percent increase in the chances of a live birth when eggs were used that had been collected during days that had the most sunshine compared to those with the least sunshine. Results have been published in Human Reproduction.
'It's long been known that there is seasonal variation in natural birth rates around the world, but many factors could contribute to this including environmental, behavioural and sociological factors,' said lead author Dr Sebastian Leathersich, an obstetrician, gynaecologist and fellow in reproductive endocrinology and infertility at Fertility Specialists of Western Australia.
'Most studies looking at IVF success rates have looked at fresh embryo transfers, where the embryo is put back within a week of the egg being collected. This makes it impossible to separate the potential impacts of environmental factors, such as season and hours of sunshine, on egg development and on embryo implantation and early pregnancy development.'
The increasing use of frozen embryo transfers provided an opportunity to investigate the potential impact of time of egg collection, he added.
In the study, Dr Leathersich and his team examined the impact of season, temperature, and sunlight during egg retrieval and embryo transfer on pregnancy outcomes in Perth, Western Australia.
They analysed 3659 frozen embryo transfers with embryos generated from 2155 IVF cycles in 1835 patients over an eight-year period and reported that those embryos created from eggs collected in the summer months have a 30 percent increase in live birth compared to those created during autumn.
Authors suggested mechanisms behind the finding could include vitamin D, melatonin levels, pollution and the impact of these variables in the months during gamete development. Dr Leathersich told the Times the study demonstrated the potential role of environmental factors on egg and embryo quality, though he did not know why this might be. 'I certainly don't think people should cancel winter IVF cycles', he said.
Ying Cheong, professor of reproductive medicine at the University of Southampton who was not involved in the research also advised against rushing into summer treatments. 'Such associative data is useful and interesting, however fertility treatment requires careful planning and patients should prioritise their clinical and psychological readiness over seasonal factors'.