By Carly Cassella

There's something about the seasons that seems to impact human reproduction for unknown reasons.

A new study in Australia has found in vitro fertilization (IVF) is most successful when eggs are harvested in the summer.

It doesn't seem to matter when the frozen embryo is actually transferred to a person's womb, only when it is originally collected.

If eggs are retrieved in summer, the live birth rate is 31 percent; if eggs are collected in autumn, the live birth rate is 26 percent.

Meanwhile, eggs collected in winter and spring sit between these two success rates.

While previous IVF studies found seasons have no consistent impact on embryo transfers, implantation, pregnancy, or live birth rates, egg collection seems to be a different matter.

"Most studies looking at IVF success rates have looked at fresh embryo transfers… " explains King Edward Memorial Hospital ob-gyn Sebastian Leathersich.

"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."

Researchers analyzed data from eight years and 3,657 frozen embryo transfers performed by a single fertility clinic in Perth.

Of all these embryos, those collected on days with more than 10 hours of sunshine were 28 percent more likely to result in a live birth than those collected when the sun showed its face for less than 7 hours a day.

However, the actual temperature of the day didn't seem to have an impact.

The research was done in hindsight, so it can't reveal a direct cause and effect.

That said, a similar 2022 study in the Northern Hemisphere found that the season and temperature at the time of egg retrieval significantly impacted the subsequent live birth rate.

Specifically, eggs collected during summer in Boston were 42 percent more likely to result in a live birth than those collected during winter. Meanwhile, eggs collected on the warmest days were 34 percent more likely to result in a live birth than eggs collected on the coldest days.

The findings suggest that seasons can have an important effect on a person's ovarian function, although not necessarily the receptiveness of their uterus or the early development of a fetus.

The two studies disagree, however, on whether ambient temperature or the duration of bright sunshine is the more important seasonal factor impacting embryo retrieval, possibly via vitamin D or the production of melatonin.

Maybe it's neither.

"It is possible that there are differences in activity, diet, and lifestyle in different seasons which could underlie the observed differences in live birth rates, though such data were not collected in this study," the authors acknowledge.

"It is also possible that other environmental factors, including pollutants, may impact clinical outcomes."

Even though plenty of questions remain left to be answered, given the results, researchers say patients may opt to collect eggs in the summer months when daylight hours are higher.