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IVF success rates from frozen embryos vary depending on the time of year eggs are collected, with a 30% higher chance of live birth when egg collection occurs in summer compared to autumn, new Australian research finds.
Publishing in Human Reproduction, Western Australian researchers analysed outcomes from 3,659 frozen embryo transfers (from 2,155 IVF cycles in 1835 patients) at a single clinic in Perth between January 2013 and December 2021.
The average live birth rate across the sample was 28 births per 100 people.
If eggs were collected in autumn, the rate was 26 births per 100 people, compared to 31 births per 100 people in summer. This seasonal improvement in birth rates was seen regardless of when the embryos were finally transferred to the women’s wombs.
Obstetrician and gynaecologist Dr Sebastian Leathersich who led the study says the new research separates out the effect of environmental factors at the time of egg collection from the time of embryo transfer.
The new research analyses the effect of environmental factors – season, temperature and recorded hours of sunlight exposure – at the time of egg collection, and the transfer time, drawing on data from the Australian Bureau of Meteorology.
In Australia, over 60% of embryo transfers are from frozen eggs, Leathersich says. The average live birth rate from IVF nationally is 27 births per 100 people.
He emphasises that the study uses retrospective data, and investigates associations, rather than causation.
“But what we found was that those eggs that have been collected in summer and then frozen as embryos, if they were transferred, they had a higher chance of being born as a live baby than eggs that were collected in in autumn.”
The research also found a 28% increase in the chances of a live birth among women who had eggs collected during days that had the most sunshine (high sunshine days were classified as having 10.7 to 13.3 hours of sunshine) compared to days with the least sunshine (0 to 7.6 hours).
Leathersich says possible factors explaining the higher success rates in summer could include melatonin levels during egg development, or vitamin D levels.
“Melatonin in particular is an interesting candidate. It’s a hormone involved in circadian rhythm. And some small studies have shown that it may improve embryo development if given during an IVF cycle,” he says.
Peak melatonin levels actually occur during winter. But as human eggs take somewhere between six to nine months to develop, eggs collected in summer potentially have higher exposure to melatonin.
Temperature during egg collection did not affect the rates of live births. But temperature during transfer did.
The chances of a live birth decreased by 18% when the embryos were transferred on the hottest days (average temperature of 14.5 – 27.80 degrees C) compared to the coolest days (0.1 – 9.80 degrees C), and there was a small increase in miscarriage rates, from 5.5% to 7.6%.
“We don’t know the exact mechanism for the observations […] obviously further research is required both to confirm the findings in other settings, but also to look at what the underlying cause might be.”
Leathersich adds, “I certainly wouldn’t be recommending that people run out and cancel their winter IVF cycles.”
“There are lots of factors that we know have an impact on reproductive success, most importantly, things like maternal age and also paternal age.
“Other factors that we know impact success that people can control, include avoiding smoking, minimising alcohol consumption, moderating caffeine intake, maintaining healthy, balanced diet, aiming for healthy weight and the moderate amount of exercise.”
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