IVF may be more successful if eggs are harvested in summer

The time of year when eggs are collected from women’s ovaries during fertility treatment appears to make a difference to live birth rates, according to a new study from Australian researchers. The results of the research, published in the journal Human Reproduction, found that transferring frozen then thawed embryos to women’s wombs from eggs collected in the summer resulted in a 30% higher likelihood of babies born alive than if the eggs had been retrieved in the autumn.

Until now, there have been conflicting findings on the effects of the seasons on pregnancies and live birth rates following egg collection and embryo freezing. As explained by study leader Dr Sebastian Leathersich, from King Edward Memorial Hospital, City Fertility Australia and Fertility Specialists of Western Australia, “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.

“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.

“These days, many embryos are frozen and then transferred at a later date. We realised this gave us an opportunity to explore the impact of environment on egg development and on early pregnancy separately by analysing the conditions at the time of egg collection independently from the conditions at the time of embryo transfer.”

Leathersich and his colleagues analysed outcomes from all frozen embryo transfers carried out at a single clinic in Perth over a period of eight years, from January 2013 to December 2021. During this time there were 3659 frozen embryo transfers, with embryos generated from 2155 IVF cycles in 1835 patients. Information on outcomes was missing for two frozen embryo transfers and so these were excluded, leaving 3657 for analysis.

The researchers looked at birth outcomes according to season, temperatures and the actual number of hours of bright sunshine (as opposed to calculating hours from sunrise to sunset), with weather data obtained from the Bureau of Meteorology. They created three groups for duration of sunshine on days when eggs were collected: low-sunshine days (0 to 7.6 hours of sunshine), medium-sunshine days (7.7 to 10.6 hours) and high-sunshine days (10.7 to 13.3 hours).

“When we looked specifically at the duration of sunshine around the time the eggs were collected, we saw a similar increase to that seen for egg collection during the summer,” Leathersich said. “The live birth rate following a frozen embryo transfer from an egg that was collected on a day with fewer hours of sunshine was 25.8%; this increased to 30.4% when the embryo came from an egg that was collected on days with the most hours of sunshine. When we took into account the season and conditions on the day of the embryo transfer, this improvement was still seen.”

The temperature on the day of egg collection did not affect the chances of a live birth. However, the chances of a live birth decreased by 18% when the embryos were transferred on the hottest days (average temperature of 14.5–27.8°C) compared to the coolest days (0.1–9.8°C), and there was a small increase in miscarriage rates, from 5.5% to 7.6%.

“Our study suggests that the best conditions for live births appear to be associated with summer and increased sunshine hours on the day of egg retrieval,” Leathersich said.

“In our study, the overall live birth rate following frozen embryo transfer was 28 births per 100 people. If eggs were collected in autumn, it was 26 births per 100 people, but if they were collected in summer there were 31 births per 100 people. This improvement in birth rates was seen regardless of when the embryos were finally transferred to the women’s wombs. The live birth rates when eggs were collected in spring or winter lay between these two figures.”

Factors that may play a role in the increased live birth rates after egg collection in the summer and during more sunshine hours include melatonin. Levels of this hormone are usually higher in winter and spring, and eggs take three to six months to develop before they are released from the ovaries. Differences in lifestyles between winter and summer months may also play a role.

The findings that miscarriage rates were highest when embryo transfer took place on the hottest days are consistent with epidemiological studies that show higher rates of miscarriage in the summer months. According to Leathersich, “This suggests that the negative effects of high temperature are more likely related to early pregnancy rather than egg development.”

Leathersich acknowledged that there are many factors that influence fertility treatment success, including age, avoiding smoking, alcohol and other toxins, and maintaining healthy activity levels and weight. However, he said, this study suggests clinicians and patients could also consider the importance of environmental factors and their influence on egg quality and embryonic development.

“We effectively separated the conditions at the time of egg collection from the conditions at the time of transfer, demonstrating that environmental factors when the eggs are developing are as, if not more, important than environmental factors during implantation and early pregnancy,” he said.

Leathersich also recognised that it was a retrospective rather than prospective study, and so it cannot show that conditions at the time of egg collection cause the difference in live birth rates — only that they are associated with them. He said the research should ideally be replicated in other sites with different conditions and different treatment protocols in order to confirm the findings.

“It would also be interesting to look at the impact of season and environmental factors on sperm parameters, as this could have contributed to our observations,” he said. “We are now planning to analyse this same group of patients using air quality data, as there may be seasonal changes in exposure to harmful pollutants which could negatively affect reproductive outcomes.

“Finally, given the huge increase in so-called ‘social egg freezing’ for fertility preservation and the fact that this group generally have flexibility about when they choose to undergo treatment, it would be very interesting to see if these observations hold true with frozen eggs that are thawed and fertilised years later. Any improved outcomes in this group could have big impacts for women making decisions about their future fertility, but the long-term follow-up required means it is likely to be some time before we can draw any conclusions for this population.”

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