IVF may be more effective if eggs are harvested in the summer

According to a new study by Australian researchers, the time of year when eggs are retrieved from women’s ovaries during fertility treatment appears to make a difference in live birth rates. The results of the research, published in the journal human reproductionfound that transferring frozen and then thawed embryos into women’s wombs from eggs collected in the summer resulted in a 30% higher likelihood of babies being born alive than if the eggs were retrieved in the fall.

So far, there have been conflicting results on the effects of seasons on pregnancies and live birth rates after egg collection and embryo freezing. As explained by the study director, Dr. Sebastian Leathersich, of King Edward Memorial Hospital, City Fertility Australia And Western Australia Fertility Specialists“It has long been known that there are seasonal variations in natural birth rates around the world, but many factors could contribute to this, including environmental, behavioral and sociological factors.

“Most studies looking at IVF success rates have focused on fresh embryo transfers, where the embryo is put back in place within a week of egg collection. This makes it impossible to separate the impacts potential environmental factors, such as season and hours of sunlight, on egg development and on embryo implantation and early pregnancy development.

“Nowadays, many embryos are frozen and then transferred later. We realized that this gave us the opportunity to separately explore the impact of the environment on egg development and on the onset of pregnancy by analyzing the conditions at the time of egg collection independent of the conditions at the time of egg collection. embryo transfer.

Leathersich and colleagues analyzed the results of all frozen embryo transfers performed at a single clinic in Perth over an eight-year period, from January 2013 to December 2021. During this time there were 3,659 embryo transfers frozen, with embryos generated from 2155 IVF cycles. in 1835 patients. Outcome information was missing for two frozen embryo transfers and these were therefore excluded, leaving 3657 for analysis.

Researchers looked at birth results based on season, temperatures, and actual hours of sunshine (as opposed to calculating sunrise to sunset times), with weather data obtained from the Bureau of Meteorology . They created three groups for the sunshine duration on the days the eggs were collected: days with low sunshine (0 to 7.6 hours of sunshine), days with medium sunshine (7.7 to 10.6 hours) and days with strong sunshine (10.7 to 13.3 hours).

“When we looked specifically at the sunshine duration at the time of egg collection, we found 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 collected on a day with fewer hours of sunlight was 25.8%; this figure rose to 30.4% when the embryo came from an egg collected on the sunniest days. When we took into account the season and the conditions on the day of the embryo transfer, this improvement was still visible.

The temperature on the day of egg collection did not affect the chances of live birth. However, the chances of live birth decreased by 18% when embryos were transferred on the hottest days (mean temperature 14.5 to 27.8°C) compared to the coolest days (0.1 to 9 .8°C), and there was a slight increase. miscarriage rate, 5.5% to 7.6%.

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

“In our study, the overall live birth rate after frozen embryo transfer was 28 births per 100 people. If the eggs were collected in the fall, there were 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 into the wombs. The live birth rates when the eggs were collected at the spring or winter fall between these two figures.

Factors that may play a role in increasing live birth rates after egg collection in the summer and during more hours of sunlight include melatonin. Levels of this hormone are generally highest in winter and spring, and eggs take three to six months to develop before being released from the ovaries. Lifestyle differences between the winter and summer months may also play a role.

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

Leathersich acknowledged that many factors affect the success of fertility treatment, including age, avoiding smoking, alcohol and other toxins, and maintaining activity and fitness levels. a healthy weight. However, he said, this study suggests that 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 during egg development are as, if not more, important than environmental factors during implantation and onset. pregnancy,” he said. .

Leathersich also acknowledged that this was a retrospective study rather than a prospective one and therefore cannot show that conditions at the time of egg collection cause the difference in live birth rates – only that they are associated with it. He said the research should ideally be replicated at other sites with different conditions and different treatment protocols to confirm the results.

“It would also be interesting to examine the impact of season and environmental factors on sperm parameters, as this could have contributed to our observations,” he said. “We now plan to analyze this same group of patients using air quality data, as there may be seasonal changes in exposure to harmful pollutants that 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 has flexibility in 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 fertilized years later.Any improvement in outcomes in this group could have a big impact on women making decisions about their future fertility, but the long-term follow-up required means it will likely take some time before we can draw any conclusions for this population.

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