Association between season and pregnancy outcomes in fresh embryo transfer cycles: a systematic review and meta-analysis.

BACKGROUND

Infertility represents a major global public health challenge. Identifying modifiable factors to optimize the outcomes of assisted reproductive technology (ART) is therefore crucial. Seasonal exposure has been reported to be correlated with natural conception rates, birth patterns, and adverse pregnancy outcomes. However, the potential associations between seasonal exposure and pregnancy outcomes among women undergoing fresh embryo transfer cycles remain controversial. This study aims to determine whether an association exists between seasonal exposure and pregnancy outcomes following fresh embryo transfer cycles, thereby providing evidence-based guidance on whether seasonal considerations should be integrated into ART treatment planning.

METHODS

This review adhered to the PRISMA guidelines. Literature searches were conducted across seven major electronic databases. Data synthesis employed either fixed-effects models. Subgroup analyses were performed according to the Köppen climate classification. The risk of bias in the included studies was assessed using the Newcastle-Ottawa Scale (NOS). Associations are reported as odds ratios (ORs) with corresponding 95% confidence intervals (CIs), and heterogeneity was quantified using the I statistic.

RESULTS

A total of 19 retrospective studies encompassing 159,696 fresh embryo transfer cycles were included. Overall, no significant associations were found between season and clinical pregnancy or live birth. Subgroup analyses, however, revealed climate-specific variations: In Cwa (humid subtropical) climates, live birth rates were significantly higher in summer than in winter (OR = 1.05; 95% CI, 1.01-1.10; I = 0%, P heterogeneity < 0.05). In Dfb (warm-summer humid continental) climates, live birth rates were significantly higher in spring than in summer (OR = 1.07; 95% CI, 1.01-1.14).

CONCLUSION

Seasonal variation exhibits no significant association with pregnancy outcomes following fresh embryo transfer cycles. The observed variations within specific climate subgroups may result from multifactorial influences and require further research for validation. Clinicians and patients can prioritize medical indications and personal readiness without concern that seasonal timing may adversely impact pregnancy outcomes.

SYSTEMATIC REVIEW REGISTRATION

https://www.crd.york.ac.uk/PROSPERO/view/CRD420251077751, Identifier: CRD420251077751.