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

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

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

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.

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.

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.

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.