Are we more fertile in the summer?

The key to IVF success is to collect the eggs in summer, and implant them on cooler days, new research suggests.

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IVF is more successful when egg retrieval happens in summer, and if embryo transfer happens on cooler days, according to Australian research.

“Optimal conditions for live birth appear to be associated with summer and increased sunshine hours on the day of oocyte retrieval, rather than the average temperature on the day of oocyte retrieval,” said the researchers, led by Dr Sebastian Leathersich, a gynecologist from King Edward Memorial Hospital.

“In contrast, high minimum temperatures on the day of frozen embryo transfer are associated with reduced live births.”

The retrospective cohort study, published in Human Reproduction, reviewed the season, temperature and hours of bright sunshine at egg collection and embryo transfer in almost 2000 patients who underwent IVF at a clinic in Perth in the last decade.

The data showed that the odds of a live births were 30% higher when eggs were collected in summer compared to autumn, and 28% higher on days with high hours of daily sunshine compared with low hours.

Implanting the embryo in different seasons or in different weather conditions didn’t affect the summer and sunshine benefits.

Nor did the temperature on the day of egg retrieval.

Previous research has suggested melatonin levels may be involved in the maturation of eggs. But the researchers said this was unlikely to be responsible for the summer and sunshine boosts, because higher peak level and secretion duration of melatonin occured in winter.

Vitamin D – shown to be lowest in winter in an Australian population – was also unlikely to be involved as the odds of live births were lowest when eggs were collected in autumn.

The authors also found that the hours of sunshine in the fortnight and month prior to egg collection did not affect odds of a live birth, suggesting the driver was not sunshine exposure during folliculogenesis.

Instead, the authors suggested seasonal lifestyle or environmental variations, such as exercise and pollutant levels respectively, may underlie the variation.

On the other hand, the rate of live births was unaffected by the season that embryo transfer occurred in, and did seem to be affected by the temperature at the time of embryo transfer. If the day did not cool down and the minimum temperature was high, odds of live birth decreased by 18% compared to cooler nights with a low minimum temperature at time of transfer.

Similarly, the risk of miscarriage was also higher if the maximum daily temperature was high at transfer compared to days with a low maximum temperature.

“Interestingly, our data suggest that this association persists even when corrected for the conditions at the time of oocyte collection, suggesting that it is high temperatures in early pregnancy rather than during oocyte development that underlie this finding,” the authors said.

“Given the increasing utilisation of cryopreservation and frozen embryo transfers, patients and clinicians may opt to collect oocytes and cryopreserve embryos in the summer months when daylight hours are higher with a view to increasing live birth rates per embryo transfer.”

“Our findings suggest that seasonal or environmental conditions affect oocyte development and/or maturation, and ultimately embryo competence and subsequent live birth rates, rather than impacting on uterine receptivity and early pregnancy development.”

The study did not independently assess the effects of season or environmental condition on sperm number or the resulting effects on live births.

Human Reproduction, online 5 July