Previous research suggests seasonal variation may influence fertility and pregnancy outcomes, but underlying hormonal mechanisms remain unclear. This analysis examined seasonal variation in reproductive hormones—luteinizing hormone (LH), follicle-stimulating hormone (FSH), estradiol (E2), and progesterone (P4)—using data from the EAGeR trial (2006–2012) and the BioCycle study (2005–2007).

The EAGeR trial included 1,228 premenopausal women from four U.S. sites (Buffalo, Scranton, Denver, and Salt Lake City) attempting conception, with urinary hormone measurements collected at three time points per cycle. The BioCycle study enrolled 250 premenopausal women from Buffalo, NY, with serum hormone levels measured at eight time points per cycle. Seasons were categorized as spring (March–May), summer (June–August), fall (September–November), and winter (December–February). Seasonal hormone variation was assessed using mean (SD) and ANOVA.

In BioCycle, E2 levels were highest in summer during menstruation, mid-follicular, and late follicular phases. No significant seasonal E2 variation was observed in EAGeR. LH levels were highest in spring in BioCycle, particularly during menstruation (mean 4.92 [SD 3.13] vs. 3.71 [SD 1.54] ng/mL in fall). In EAGeR, LH peaked in fall across follicular, ovulatory, and luteal phases. FSH was highest in winter in BioCycle and fall in EAGeR. No consistent seasonal pattern was observed for P4.

Overall, seasonal differences in reproductive hormone levels were observed, with higher LH and FSH levels generally occurring during cooler months and higher E2 levels during warmer months. These findings highlight the need for further research to clarify mechanisms linking seasonal hormonal variation to reproductive health outcomes.