The time of year when eggs are collected from women’s ovaries during fertility treatment influences live birth rates, according to recent research published in Human Reproduction, one of the world’s leading reproductive medicine journals.

The Australia-based research team found that transferring embryos, initially frozen then thawed, which were retrieved from eggs collected in the summer, resulted in a 30% higher likelihood of live births than those collected in the autumn.

Dr Sebastian Leathersich, who led the study, stated, "If eggs were collected in autumn, the live birth rate was 26 births per 100 people, but if collected in summer there were 31 births per 100 people. This improvement was seen regardless of when the embryos were finally transferred to the women’s wombs."

The study also observed a 28% increase in the chances of a live birth among women who had eggs collected on days with the most sunshine compared to those with the least sunshine.

New Insights into Seasonal Variations

Previous studies have presented conflicting findings about the influence of seasons on pregnancies and live birth rates after egg collection and embryo freezing. Dr Leathersich elaborated, "Most studies have looked at fresh embryo transfers, where the embryo is returned within a week of the egg being collected. We realised that many embryos are now 'frozen' and transferred later, allowing us to explore the impact of environment on egg development and early pregnancy separately."

The study analysed outcomes from all frozen embryo transfers carried out at a single clinic in Perth over a period of eight years. The researchers took into account factors such as the season, temperature, and actual number of hours of bright sunshine.

The results showed that the likelihood of a live birth was higher when the egg was collected on a day with more sunshine, irrespective of the season and conditions during the embryo transfer.

However, the study found that temperature on the day of egg collection did not influence live birth chances. Interestingly, live birth rates decreased by 18% when embryos were transferred on the hottest days, accompanied by a small rise in miscarriage rates.

Implications of the Findings

"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," said Dr Leathersich.

Factors that may contribute to increased live birth rates after summer egg collection and during more sunshine hours include melatonin, as its levels are typically higher in winter and spring. Lifestyle differences between winter and summer months may also play a part.

Moreover, the study findings that miscarriage rates were highest when embryo transfer occurred on the hottest days echo epidemiological studies showing higher rates of miscarriage during summer months.

Despite these promising findings, the study's retrospective nature means it can only demonstrate an association between conditions at the time of egg collection and the difference in live birth rates, rather than causation.

Looking forward, Dr Leathersich believes the findings should be confirmed through replication in different settings, with varying conditions and treatment protocols. He also expressed an interest in analysing the impact of seasonal and environmental factors on sperm parameters.

Given the surge in "social egg freezing" for fertility preservation, Dr Leathersich stated, "It would be very interesting to see if these observations hold true with frozen eggs that are thawed and fertilised years later."

While this study offers significant implications for fertility treatments, it also highlights the intricate balance between environmental factors and biological processes, and the ever-evolving understanding of reproductive medicine.

Sources & References

ESHRE - More IVF babies born after egg collection in summer rather than in autumn