Background
High parental age is associated with adverse birth and genetic outcomes, but little is known about fecundity in male offspring.

Objectives
We investigated if high parental age at birth was associated with biomarkers of male fecundity in a large population-based sample of young men.

Materials and methods
We conducted a study of 1057 men from the Fetal Programming of Semen Quality (FEPOS) cohort, a sub-cohort of sons born 1998–2000 into the Danish National Birth Cohort. Semen characteristics and reproductive hormone concentrations were measured in samples provided by the men 2017–2019. Testis volume was determined by self-measurement. Data on the parental age was drawn from registers. Adjusted relative difference in percentage with 95% confidence intervals were estimated for each outcome according to pre-specified maternal and paternal age groups (< 30 (reference), 30–34 and ≥ 35) as well as for combinations of parental age groups, using multivariable negative binomial regression models.

Results
We did not observe consistent associations between parental age and biomarkers of fecundity, although sons of mothers ≥ 35 years had lower sperm concentration (−15% (95% CI: −30, 3)) and total sperm count (−10% (95% CI: −25, 9)). The analysis with parental age combinations showed lower sperm concentration with high age of the parents (both ≥ 35 years: −27%, 95% CI: −40, −19) when compared to the reference where both parents were below 30 years.

Discussion and conclusion
We found no strong association between higher parental age and biomarkers of fecundity in young men. However, we cannot exclude poorer semen characteristics in sons born by older mothers or with high age of both parents.