Season of birth and variations in male reproductive health: A population-based cohort study

BACKGROUND: Season of birth has been associated with various later reproductive health outcomes in women, but little is known on the potential associations in men.

OBJECTIVES: To investigate the association between season of birth and semen characteristics, testes volume and reproductive hormone levels in young men.

MATERIALS AND METHODS: We conducted a follow-up study of 1058 young men, born 1998 to 2000, from the Fetal Programming of Semen Quality (FEPOS) cohort, Denmark, 2017-2019. Information on season of birth was obtained from the Danish Civil Registration System, and information on male reproductive health outcomes was obtained at a clinical examination, where the men provided a semen and a blood sample and measured testes volume. Percentage differences in semen characteristics, testes volume and reproductive hormone levels were calculated according to season of birth (binary (main analysis): summer; winter and categorised by four calendar seasons and by calendar month (subanalyses)) using adjusted regression models and visualisalised according to month of birth.

RESULTS: Testosterone levels were lower (-3% (95% CI: -7%; 0%)) and oestradiol levels were higher (10% (95% CI: 2%; 20%)) in men born during the winter half-year than the summer half-year. The finding of higher oestradiol in men born during the winter was corroborated in analyses of calendar season and month of birth. Other reproductive health outcomes displayed some variation; however, estimates were generally close to null.

DISCUSSION: Although oestradiol levels seemed higher in men born during the winter half-year, this could be a chance finding. Since pregnancies usually span three seasons, this finding could therefore also reflect an association between early pregnancy during the summer and oestradiol levels.

CONCLUSION: We observed higher oestradiol levels in men born during the winter than during the summer half-year. For the remaining reproductive health outcomes, the observed fluctuations may reflect random variation.