Rotating shiftwork in men is linked to decreased fecundity, attributable primarily to sperm concentration and vitality: 24-month follow-up in 3425 males of PREBIC cohort.

Background: Previous observational studies investigating the association between shift work and male fecundity employed retrospective designs and reported inconsistent findings. The distinction between rotating and fixed shift work has been noticed but not often taken into consideration in the field of reproductive health. Whether semen parameters played a role in this relationship was also unclear.

Objective: This study aimed to prospectively evaluate the association between male rotating shift work and achieving a successful pregnancy, and to assess the contribution of semen parameters to this relationship.

Materials and methods: Men reported their work type (rotating shift workers, fixed day workers, and fixed shift workers) and seven semen parameters (semen volume, sperm concentration, total sperm count, progressive motility, total motility, vitality, and normal morphology) were measured. Participants were contacted every 3 months for pregnancy status for up to 24 months or until conception. The rates of achieving a successful pregnancy within 1 year and 2 years were calculated to estimate male fecundity. Cox regression and linear regression were applied to compare the hazard of achieving a pregnancy within 1/2 year and semen parameters among rotating shift workers, fixed day workers, and fixed shift workers. Mediation analysis was applied to examine the contributions of semen parameters to the shiftwork-fecundity association.

Results: (1) After adjusting confounders, rotating shift workers had a 16% (HR = 0.84, 95% CI, 0.70-0.99, p = 0.04) and 14% (HR = 0.86, 95% CI, 0.74-1.01, p = 0.06) lower hazard of achieving a pregnancy within 1 year and 2 years, respectively, compared to fixed day workers. (2) Of the seven semen parameters, the rotating shift workers were significantly associated with lower sperm concentration (β = -10.77%, 95% CI, -18.89% to -2.55%, p = 0.01), total sperm count (β = -11.63%, 95% CI, -21.25% to -2.02%, p = 0.02), and vitality (β = -3.86%, 95% CI, -7.00% to -0.72%, p = 0.02) compared to the fixed day workers. No significant difference was found between fixed day workers and fixed shift workers. (3) Sperm concentration accounted for 33.46% and 31.12% of the total effect of the relationship between rotating shift work and lower hazard of achieving a pregnancy within 1 and 2 years. Vitality also mediated this association by 30.79% and 23.59%, respectively.

Conclusions: Male rotating shift work was associated with reduced fecundity, primarily attributed to reduced sperm concentration and vitality. The study emphasizes the need for occupational protection and provision of medical care for male workers with rotating shifts.