Every claim, with a citation.
This page indexes the primary sources behind FutureKit's claims about male fertility — eleven studies, meta-analyses and clinical guidelines, grouped by topic, each with a one-paragraph takeaway and a direct link to PubMed or the journal. Where a study is cited in a FutureKit blog post, the entry lists the post inline.
Male fertility is a young clinical literature. Most of what we say on this site rests on a small set of well-conducted studies and consensus guidelines. This page is the index — every claim worth questioning links back to its source.
Population-level decline
- Levine et al., 2017Human Reproduction Update
Meta-analysis of 244 studies found a substantial decline in sperm concentration and total sperm count among men in Western countries between 1973 and 2011.
Paternal age
- Kong et al., 2012Nature
Whole-genome sequencing of Icelandic trios estimated approximately two additional de novo germline mutations per year of paternal age.
- Johnson et al., 2015Ageing Research Reviews
Systematic review showed semen volume, sperm motility and normal morphology decline with age, even in men reporting good general health.
- du Fossé et al., 2020Human Reproduction Update
Meta-analysis found advanced paternal age (≥45) associated with a roughly 30% relative increase in miscarriage risk after adjustment for maternal age.
TRT and fertility
- Hayes & Hodson, in WHO contraception trials review, 2017Asian Journal of Andrology
Reviews of male hormonal contraception trials show exogenous testosterone induces azoospermia or severe oligozoospermia in roughly 95–98% of men within six months at full replacement doses.
- Liu et al., 2006The Lancet
Pooled analysis of male contraceptive trial data showed median time to recovery of sperm production after stopping exogenous testosterone is approximately six months, with a long tail.
Lifestyle effects
- Leisegang & Dutta, 2019Andrologia
Review consolidates evidence that obesity, smoking, alcohol, heat exposure and chronic stress impair sperm parameters and that lifestyle modification produces measurable improvements within one spermatogenesis cycle.
- Ricci et al., 2018Reproductive BioMedicine Online
Systematic review found regular cannabis use associated with lower sperm concentration and motility, with effects most pronounced at high frequency of use.
Diagnostics & guidelines
- Agarwal et al., 2019World Journal of Men's Health
Review establishes DNA fragmentation index (DFI) as an independent predictor of pregnancy outcomes; DFI explains a portion of unexplained infertility cases that pass standard semen analysis.
- WHO laboratory manual, 6th ed., 2021World Health Organization
Sets the current reference values for human semen analysis (concentration ≥16 M/mL, total motility ≥42%, progressive motility ≥30%, normal morphology ≥4%).
- ESHRE guideline group, 2018ESHRE Guidelines
European reproductive medicine guidance on workup of male and couple infertility, including indications for hormone testing and semen analysis.
Get your own baseline.
The Hormone Panel 01 measures the six markers most relevant to the literature above — Total + Free Testosterone, FSH, LH, SHBG, Prolactin, Estradiol.