Impact of Seasonal Variations on IVF Pregnancy Outcomes: A Retrospective Cohort Study in Jinan, Shandong Province, China

Xin Hu¹Hengbing Li¹Jing-Yan Song²海宁 袁¹Ying Xu¹Kai-Liang Ai¹Zhen-Gao Sun^2*Zhenni Mu^2*

• ¹Shandong University of Traditional Chinese Medicine, Jinan, China
• ²Shandong University of Traditional Chinese Medicine Affiliated Hospital, Jinan, China

Background: To evaluate the association between seasonal temperature variations and clinical outcomes of in vitro fertilization (IVF), aiming to provide theoretical foundations for optimizing protocol timing in reproductive medicine. Methods: This retrospective cohort study analyzed 2,551 first fresh IVF-embryo transfer (IVF-ET) cycles performed at a tertiary reproductive center of The First Affiliated Hospital of Shandong University of Traditional Chinese Medicine between January 2009 and January 2024. The study population comprised normo-ovulatory women aged <35 years without uterine anomalies or severe male factor infertility (sperm concentration >1×10⁶/mL). Cycles were stratified into four seasonal cohorts based on gonadotropin initiation dates: spring (March-May, n=709), summer (June-August, n=787), autumn (September-November, n=640), and winter (December-February, n=415). Primary outcomes included clinical pregnancy rate (CPR), live birth rate (LBR), miscarriage rate (MR), and Full-Term delivery rate (FTBR), analyzed through multivariable logistic regression models adjusting for mean daily temperature (°C), relative humidity (%), and daylight hours (h). Results: Compared with the winter control group, risk of miscarriage in cycles initiated in spring showed a statistically significant increase (95% CI 1.019, 2.846; P=0.042). Although CPR showed no seasonal variation (spring: 54.30%, summer: 52.22%, autumn: 50.47%, winter: 50.36%; P=0.464), the spring cohort exhibited a numerically higher Full-Term delivery rate (39.07% vs 34.22%; P=0.105). Sensitivity analysis using weighted analysis to balance sample sizes across groups revealed significantly higher full-term birth rates in spring compared to winter (P=0.046) and the live birth rate in spring was also significantly higher than in winter (P=0.029). For each unit increase in sunlight intensity on the trigger day, the probability of successful pregnancy decreases to approximately 0.978 times the original value (OR=0.978 per 42 Conclusion: Seasonal microenvironmental factors during ovarian stimulation may 43 modulate IVF success trajectories, suggesting potential benefits of climate-adaptive 44 protocol personalization in temperate monsoon regions.