Correia et al. (2022) and Leathersich et al. (2023) investigate the association of IVF outcomes following egg retrieval during different seasons, and find that retrievals in the summer resulted in the best outcomes.
The impact of the seasons on IVF outcomes is inconclusive, with some studies showing associations while others find no differences. Importantly, most studies have looked at outcomes at the time of embryo transfer, and not egg collection.
This post will discuss two independent studies that explore the potential links between seasonal timing of egg retrievals and its impact on frozen embryo transfer (FET) outcomes. The Correia et al. (2022) study was based in the Northern Hemisphere (Boston, USA), and the Leathersich et al. (2023) study was based in the Southern Hemisphere (Perth, Australia).
Seasons were defined meteorologically as summer (December–February), spring (March–May), winter (June–August), and autumn (September–November).
For temperature, both studies categorized it by tertiles (divided into three parts). The Boston study used historical data for temperature (<6.7°C, 6.7–17.2°C, and >17.2°C), while the Perth study actually collected the average, maximum and minimum temperature for each day:
- Average temperature (°C): low: 7.9–15.5, medium: 15.6–20.9, high: 21.0–33.9
- Maximum temperature (°C): low: 13.2–21.2, medium: 21.3–27.4, high: 27.5–43.3
- Minimum temperature (°C): low: 0.1–9.8, medium: 9.9–14.4, high: 14.5–27.8
Both studies were retrospective cohort studies that took place between 2012 and 2017 (Boston) or 2013 and 2021 (Perth).
🔗 Original studies are referenced in this post or within the linked Remembryo posts.
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Seasonal IVF outcomes in the Northern Hemisphere
The Correia et al. (2022) study was based in Boston, USA, which included 3,004 FETs from 1,937 patients.
They looked at odds ratios based on season, average daily temperature and day length. The odds ratios were statistically adjusted for age, quadratic age (age squared, to account for the non-linear relationship between age and fertility), and for multiple cycles from the same patient.
Season. Compared to egg retrievals in winter, FETs using embryos produced from egg retrievals in summer showed a 51% higher odds of implantation, a 45% higher odds of clinical pregnancy, and a 42% higher odds of live birth (adjusted odds ratio [95% CI]: 1.51 [1.22-1.86], 1.45 [1.15-1.82], 1.42 [1.13-1.79], respectively). There were no differences in miscarriage rates.
Average daily temperature. Compared to average temperatures <6.7°C, FETs using embryos produced from egg retrievals with an average temperature >17.2°C showed a 54% higher odds of implantation, a 41% higher odds of clinical pregnancy, and a 34% higher odds of live birth (adjusted odds ratio [95% CI]: 1.54 [1.29-1.84], 1.41 [1.16-1.71], 1.34 [1.10-1.62], respectively). There were no differences in miscarriage rates.
Day length. Compared to day length <10.81 h, FETs using embryos produced from egg retrievals during a day length ≥13.65 h showed a 26% higher odds of implantation (adjusted odds ratio [95% CI]: 1.26 [1.06-1.51]). There were no differences in clinical pregnancy, live birth and miscarriage rates.
They also did an analysis on the timing of the FET with season, average temperature and day length. They found no statistically significant associations for implantation, clinical pregnancy, live birth and miscarriage rates.
Seasonal IVF outcomes in the Southern Hemisphere
The Leathersich et al. (2023) study was based in Perth, Australia, and included 3,659 FETs from 1,835 patients.
Like the previous study, they looked at odds ratios based on season, average daily temperature and day length. The odds ratios were statistically adjusted for age, quadratic age, and for multiple cycles from the same patient.
Season. Compared to egg retrievals in autumn, FETs using embryos produced from egg retrievals in summer demonstrated a 30% higher odds of live birth (adjusted odds ratio [95% CI]: 1.30 [1.04-1.62]). There were no differences in clinical pregnancy or miscarriage rates.
Temperature. The odds of miscarriage decreased 30% when the egg retrieval was done on a day when average temperatures were 15.6-20.9°C compared to 7.9–15.5°C (adjusted odds ratio [95% CI]: 0.70 [0.49-0.99]). They also found a 34% decrease in the odds of miscarriage when the maximum temperature was 21.3-27.4°C compared to 13.2-21.2°C (adjusted odds ratio [95% CI]: 0.66 [0.47-0.93]). There were no differences in live birth or clinical pregnancy rates.
Day length. Compared to a day length of 0-7.6 h (low sunshine hours), FETs using embryos produced from egg retrievals during a day length of 10.7-13.3 h (high sunshine hours) showed a 28% higher odds of live birth (adjusted odds ratio [95% CI]: 1.28 [1.06-1.53]). There were no differences in clinical pregnancy or miscarriage rates.
They also did an analysis on the timing of the FET with season, average temperature and day length:
- The odds of live birth decreased 18% when the FET was done on a day when minimum temperatures were 14.5-27.8°C compared to 0.1-9.8°C (adjusted odds ratio [95% CI]: 0.82 [0.69-0.99]).
- The odds of miscarriage increased 42% when the FET was done on a day when maximum temperatures were 27.5-43.3°C compared to 13.2-21.2°C (adjusted odds ratio [95% CI]: 1.42 [1.02-1.98]).
Conclusions
In terms of season, both studies found improved odds of live birth when an egg retrieval took place in the summer.
For average temperature, only the Boston study found an improvement in live birth rate when the egg retrieval took place during a period of higher temperatures (>17.2°C) compared to lower temperatures (<6.7°C). The temperatures in Boston get considerably lower than in Perth, which may account for these differences. Temperature had an impact on miscarriage rates in the Perth study.
For day length, the Boston study found no difference in live birth rates, but the Perth study found an improvement when egg retrievals took place on days that had more sunlight hours.
A separate analysis on FET timing, rather than egg retrieval timing, found mostly no associations with season, temperature and day length. However, the Perth study did find differences in live birth and miscarriage rates due to temperature.
Based on this research, it appears that seasonal or environmental factors have an impact on egg or embryo quality. Eggs develop over a period of months before ovulation, and these studies suggest that egg development in the colder months before summer may improve egg quality.
The authors addressed vitamin D, as levels of this nutrient tend to be highest in the summer months. They note that there has been conflicting evidence on its influence on IVF outcomes. The Perth study states that the lowest live birth rates were in autumn, suggesting that vitamin D doesn’t play a role (since it would be highest in the summer). The Boston study found no association with day length, but did with temperature. However, neither study measured vitamin D levels so its influence is unclear.
However, it’s not clear if seasonal differences in activity, diet, lifestyle, pollutants and sperm quality are contributing to these results, and more studies must be done to account for these factors.
References
Correia KFB, Farland LV, Missmer SA, Racowsky C. The association between season, day length, and temperature on clinical outcomes after cryopreserved embryo transfer. Fertil Steril. 2022 Mar;117(3):539-547. doi: 10.1016/j.fertnstert.2021.11.014. Epub 2021 Dec 20. PMID: 34949454.
Leathersich SJ, Roche CS, Walls M, Nathan E, Hart RJ. Season at the time of oocyte collection and frozen embryo transfer outcomes. Hum Reprod. 2023 Sep 5;38(9):1714-1722. doi: 10.1093/humrep/dead137. PMID: 37407029.
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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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