A study of more than 58,000 IVF/ICSI patients looked beyond the usual predictors of ovarian response

Age, ovarian reserve, and stimulation protocols are among the most familiar factors associated with oocyte yield during IVF.

A recent study asked a less obvious question:

Could the temperature patients are exposed to before oocyte retrieval matter too?

Researchers analyzed 58,468 women undergoing their first IVF or ICSI cycle between 2018 and 2024, examining the relationship between ambient temperature before oocyte retrieval and subsequent oocyte and fertilization outcomes.

Rather than simply comparing IVF outcomes across seasons, the researchers assessed temperature exposure across several biologically relevant periods—including 1 week, 2 weeks, 3 weeks, and 90 days before oocyte retrieval.

Higher temperatures were associated with slightly fewer oocytes

Across the study population, higher temperatures were associated with reductions in:

• Total oocytes retrieved
• Mature (MII) oocytes
• 2PN fertilized oocytes

For example, over the 90-day exposure period, every 1°C increase in apparent temperature was associated with approximately:

• 0.20% fewer total oocytes retrieved
• 0.17% fewer mature oocytes
• 0.19% fewer 2PN fertilized oocytes

The effect was modest, but the association remained notable given the size of the study population.

One particularly interesting finding was what did not change.

Despite the reduction in the number of oocytes retrieved, higher temperatures were not significantly associated with either normal fertilization rate or blastocyst formation rate.

In other words, the findings do not simply suggest that higher temperatures lead to “poorer-quality oocytes.”

Instead, temperature appeared to be associated primarily with the number of oocytes available earlier in the IVF process.

Could hormones partly explain the association?

The researchers also explored whether basal sex hormones might sit somewhere along the pathway between temperature exposure and oocyte yield.

Their analysis suggested that LH may partially mediate the association, while FSH and E2 were also examined as potential hormonal pathways.

This does not establish a direct biological mechanism, but it makes the study more interesting than a simple observation that IVF outcomes vary by season.

It suggests a possible pathway worth investigating further:

Ambient temperature → Hormonal response → Oocyte yield

So, should patients avoid IVF during hot weather?

No.

This was a retrospective observational study, so it cannot establish that higher temperature causes a reduction in oocyte yield.

Temperature exposure was also estimated using environmental data based on patients' residential locations.

Factors such as indoor air conditioning, time spent outdoors, and occupational heat exposure could not be fully captured.

The practical message is therefore not that IVF should be scheduled around the weather.

What makes the study interesting is the broader question it raises.

"How far should we look when studying the factors that influence an IVF cycle?"

IVF research has understandably focused on patient characteristics, ovarian reserve, treatment protocols, and laboratory conditions.

This study suggests that the picture may extend further—to the environment patients experience before treatment as well.

The observed effect is small and requires further validation, but it offers a different way to think about the many factors that may shape ovarian response.