Undescended testicles, or cryptorchidism, can affect later sperm production. This article connects childhood testicular development with semen studies and heat exposure, while distinguishing population birth rates from an individual diagnosis of male infertility.
Falling birth rates form the demographic background to this discussion. They should be distinguished from clinical measures of reproductive function.
In its World Population Prospects 2024 summary, the United Nations reported a global total fertility rate of 2.25 live births per woman, compared with 3.31 in 1990. This demographic measure summarizes childbearing patterns; it is not a test of any individual’s biological ability to conceive.
South Korea provides a striking historical example: Statistics Korea recorded a total fertility rate of 0.98 in 2018. The date matters when interpreting this figure; it should not be presented as a current reading or as evidence of infertility in every couple.
Changes in the timing and number of births raise social and medical questions, but national birth statistics alone cannot determine why a particular couple has difficulty conceiving. A clinical assessment asks a different question from a demographic comparison.
Sperm count, motility and morphology in the reported data
Infertility can involve male factors, female factors or both. The EAU guideline distinguishes a male factor in about 50% of infertile couples from a solely male factor in about 20%.
Low sperm counts can accompany male infertility. The studies discussed here do not establish how much of a population-level change in infertility is caused by falling sperm counts.
Semen volume and sperm number vary. A count of 60 million sperm in 3 mL is not a threshold that guarantees natural conception; semen analysis must be interpreted alongside the reproductive circumstances of both partners.
Sperm motility and morphology also matter in reproductive assessment. Neither a single percentage nor one apparently reassuring result establishes that natural conception or intrauterine insemination will be straightforward; the results must be interpreted together with both partners’ circumstances.
Population studies of semen quality need to be read in their own sampling and measurement context.
A Swiss cross-sectional study published in May 2019 analyzed semen from 2,523 young men recruited through military conscription, which normally takes place at ages 18–22. Among men with all three measures available, 38% met the study’s WHO reference criteria for concentration, motility and morphology. These reference values are not a dividing line between fertile and infertile men.
In that study, 17% had sperm concentrations below 15 million/mL, 25% had total motility below 40%, and 43% had fewer than 4% normal forms. The morphology analysis included 2,319 men; concentration and motility included 2,523. These are separate distributions, not results confined to the same subgroup.
For historical context, a 2017 meta-analysis estimated a 52.4% decline in sperm concentration and a 59.3% decline in total sperm count from 1973 to 2011 among men unselected by fertility in North America, Europe, Australia and New Zealand.
Those estimates describe a historical trend in the included studies. The analysis did not establish its causes, and its results should not be treated as a diagnosis or prediction for an individual man.
Cryptorchidism, germ cells and treatment timing
Undescended testicles (cryptorchidism) are associated with impaired fertility and testicular cancer risk. That does not establish them as the cause of population-wide sperm-count decline.
An undescended testis has not reached the scrotum; it may remain along the usual descent pathway. Some testes ascend after previously being scrotal.
Spontaneous descent can occur during early infancy, but is uncommon after six months.
EAU guidance recommends treatment from six months, corrected for gestational age, with orchiopexy by 12 months or by 18 months at the latest. Surgery moves the testis into the scrotum; timely treatment does not guarantee later fertility.
Germ cells are central to future sperm production. Research on undescended testes examines their number and maturation, rather than using one age-based count to predict every child’s future.
Progressive loss of germ cells and Leydig cells helps explain the emphasis on early treatment. Reduced fertility potential can involve several tissue changes, not one cell count alone.
Outcomes also differ between unilateral and bilateral cases. Bilateral disease carries greater concern for future fatherhood; an individual prognosis requires the clinical history and follow-up.
Testicular temperature and laptop heat exposure
A thermal study in 29 healthy volunteers found that sitting with the thighs together raised scrotal temperature, with additional warming when a working laptop rested on the lap. It did not compare infertility rates in office and manual workers.
That experiment measured scrotal temperature, not testosterone production. It does not establish a fixed 2°C temperature requirement for testosterone synthesis.
The laptop findings support a concern about heat exposure. They do not demonstrate that taking sitting breaks or avoiding a laptop on the lap restores sperm production or increases pregnancy rates.
Research sources and clinical guidance
EAU Guidelines on Paediatric Urology — Management of Undescended Testes; EAU Guidelines on Sexual and Reproductive Health — Male Infertility; AUA/ASRM guideline part I (2020); Rahban et al. Semen quality of young men in Switzerland: a nationwide cross-sectional population-based study (Andrology, 2019); Levine et al. Temporal trends in sperm count: a systematic review and meta-regression analysis (Human Reproduction Update, 2017); Sheynkin et al. Increase in scrotal temperature in laptop computer users (Human Reproduction, 2005). United Nations, World Population Prospects 2024 key message; Statistics Korea, Final Results of Birth Statistics in 2018.