Researchers in a 2021 study examined the association between ambient temperature and antral follicle count and found that exposure to rising temperatures was associated with a lower ovarian reserve.
There isn’t much known about how ambient temperature affects fertility in humans. But in animals? Oh yes!
Heat stress in dairy cows has been linked to reduced fertility, mostly in terms of egg development and a reduction in embryo quality. These effects are long-lasting too. Cows that are exposed to heat stress have egg quality issues months later into the cooler months. Check out the review by Roth 2017 for more info.
So how does temperature impact egg quality? It might have a detrimental effect on how the egg makes hormones, or it might damage different maternal mRNA transcripts that are involved in egg maturation. There’s also the possibility of increased ROS production, which can damage cellular components and DNA.
So we know all about cows and heat stress, but what about humans? A recent 2021 study by Gaskins et al. decided to investigate this further.
They looked at antral follicle counts (AFC) in order to quantify ovarian reserve. AFC is related to the number of eggs that can be retrieved in a cycle, with a higher AFC indicated a higher chance of more eggs (Shujie et al. 2019). They wanted to know if the AFC changed as temperatures changed with the seasons. These women were located in New England (Massachusetts, USA), where it gets hot, cold, and everything in between.
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These women had their AFC scan, and this was then related to their ambient temperature (based on where they lived) 3 months, 1 month, or 2 weeks before the scan. The idea here has to do with how the eggs and follicles develop over this time period:
- 3 months corresponds to when the follicle is in the early, preantral stage
- 1 month corresponds to the early antral/preovulatory stage
- 2 weeks corresponds to the final stages of antral follicle development
The 631 women they followed had a median AFC of 12 with a range from 1-30. The highest ambient temperatures were in July (28.2°C) and the lowest were in January (3.5°C).
They didn’t find a relationship between ambient temperature and AFC, but they did find a difference when they looked at changes in temperature 3 months before the AFC scan. They found that a 1°C increase in the average maximum temperature during the 3 months before their AFC was associated with a -1.6% lower AFC. What a mouthful! In other words, the more the temperature had increased over the last 3 months, the lower the AFC was.
In this study, the highest changes in average max temperature (from 3 months prior) were seen in April and May (10.7°C and 9.7°C), and the lowest in September and October (5.3°C and 5.9°C). So presumably, women who had their AFC scan done in April and May had lower AFCs compared to women in September and October (although this specific data wasn’t shared in the study).
So it wasn’t so much about the high temperature, but more about the increase in temperature, from colder to warmer, that had an impact. When the seasons change and temperatures begin to rise, the body needs to adjust. This acclimatization can create physiological stress in the body that might have an impact on follicular development. Other studies on mortality (Lee et al. 2014) and lung function (Rice et al. 2019) have found the same kind of temperature-dependent relationship.
And what about the changes after 1 month, or 2 weeks? They still found an impact (-0.9% for 1°C with 1 month and -0.8% for 1°C with 2 weeks), but this was lower than the 3 month period.
There were some weaknesses with this study that they addressed. For one, they took temperatures of the area of their residence, rather than personal temperature measurements (where they could have monitored temperatures while they traveled to work, etc.). They also lacked information about viral infections which are more common in winter months and might have had an effect.
Definitely an interesting study overall, but it’s hard to know what’s really causing this, and whether or not this can be applied to things like using a sauna, or a hot tub while trying to conceive during the colder months. Repeating this experiment in areas of the US other than the Northeast would be helpful, particularly where temperatures are more steady. It would also be nice to see if AFC counts changed with different months in the same individual, or how this relates to egg retrieval numbers.
Reference
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About Embryoman
Embryoman (Sean Lauber) is a former embryologist and the founder of Remembryo, an IVF research and fertility education website. After working in an IVF lab in the US, he returned to Canada and now focuses on making fertility research more accessible. He holds a Master’s in Immunology and launched Remembryo in 2018 to help patients and professionals make sense of IVF research. Sean shares weekly study updates on Facebook, Instagram, and Reddit regularly. He also answers questions on Reddit or in his private Facebook group.
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