Preview for Kara’s review. Expanded educational draft · sources checked 12 October 2026 · clinical approval pending.
How sperm are made, and where they go
Sperm and semen are different things. Sperm are reproductive cells carrying genetic material. Semen is the fluid ejaculated from the penis: it contains sperm plus secretions from glands, particularly the seminal vesicles and prostate. The amount of fluid alone cannot tell you how many sperm it contains.
Sperm production, called spermatogenesis, happens in tiny seminiferous tubules inside the testes. Early cells develop and divide, eventually forming sperm with a head, midpiece and tail. The head holds DNA and an acrosome; the midpiece contains energy-producing structures; the tail helps movement.
- Spermatogonium
- Primary spermatocyte
- Secondary spermatocyte
- Spermatid
- Spermatozoon
Sperm then enter the epididymis, a coiled tube where they mature and are stored. During ejaculation, they travel through the vas deferens and ejaculatory ducts, mixing with gland fluids before leaving through the urethra. Problems can arise in production, transport, or delivery.
Development and maturation take roughly two to three months. The WHO’s 2025 guideline puts sperm production at about 74 days (one small study measured about 64), and the journey through the epididymis adds roughly 2 to 14 days. So a sample today reflects a process that began about two to three months earlier. That helps explain why fever, medicines or other changes may affect later results, and why an overnight “sperm boost” is not a realistic promise.
- Early cells beginIn the testes, early cells start to divide.
- DevelopingCells take shape as sperm, then mature in the epididymis.
- Roughly 2–3 months onOnly now can a change show in a semen sample.
A sample today reflects a process that began about two to three months earlier.
Sperm take about two to three months to make and mature, so a change today can only show in a semen test months later.
Read more: WHO laboratory manual for human semen, sixth edition (2021) · WHO guideline for the prevention, diagnosis and treatment of infertility (2025)
The hormone messages behind sperm production
The brain and testes communicate through hormones. The hypothalamus releases GnRH, which stimulates the pituitary gland to release FSH and LH. FSH acts on Sertoli cells that support developing sperm. LH acts on Leydig cells, which produce testosterone.
Testosterone within the testes is important for sperm production, and the level inside the testes is many times higher than in the blood. But testosterone taken from outside the body can reduce the brain’s signals to the testes, lowering sperm production or stopping it altogether. For this reason, European (EAU) and US (AUA/ASRM) guidance advise against testosterone for men who want children now or in the future, and the UK’s NICE says it should not be offered to treat semen problems. A blood testosterone level and a sperm count answer different questions.
Tell your clinician about injections, gels, anabolic steroids and supplements, including gym products. Do not stop prescribed treatment on your own. If fertility matters now or later, ask the prescriber about assessment and alternatives appropriate to your diagnosis. If treatment is changed, sperm production usually takes months to recover; the guide Looking After Your Fertility explains the timings.
Blood tests are chosen from your symptoms, examination and semen findings. European guidance recommends measuring testosterone, FSH and LH in every man with a low count or no sperm (a strong recommendation). The UK’s NICE suggests considering these tests after two abnormal semen analyses, and US guidance uses them when the count is below about 10 million/mL or there are signs of a hormone problem. Neither a single hormone result nor a single semen result gives a complete diagnosis.
Testosterone and anabolic steroids can switch off sperm production, so they should never be used as a fertility treatment.
Read more: Endotext: Endocrinology of the testis and spermatogenesis · EAU guideline: Male infertility · NICE NG257: Fertility problems: assessment and treatment (2026) · AUA/ASRM Male Infertility guideline, amended 2024
Male age: what changes, and what the research can tell us
Kara’s research area: Kara designed research on whether a man’s age affects his sperm DNA and the chance of pregnancy in fertility treatment. The findings below come from the largest published studies.
Sperm production can continue throughout adult life, but that does not mean fertility stays unchanged. The largest review, of 90 studies and about 94,000 men, found that semen volume, sperm movement and normal shape fall gradually with age, and that DNA damage rises. Sperm numbers held up better: concentration did not fall with age.
Couples with an older male partner tend to take longer to conceive, even when the woman is young, though the effect is smaller than that of the woman’s age. In one UK study of about 2,100 pregnant women, men older than 45 were about 4.6 times more likely than men under 25 to have taken more than a year to conceive. That study included only couples who did conceive, and few of the men were older than 45.
In fertility treatment, the clearest evidence comes from cycles using donor eggs, because the eggs come from young women. Across 11 studies and about 10,500 donor-egg cycles, each extra year of the father’s age was linked with a very small, gradual fall in live-birth rates, with no sudden cut-off. IVF studies using the couple’s own eggs show bigger differences, but older men often have older partners, so part of that gap probably reflects the woman’s age. One IVF and ICSI study of 278 men found no male-age effect once the woman’s age was taken into account.
Population averages cannot predict one man’s chance of becoming a father. A study measuring sperm movement is not the same as a study measuring live births. Avoid treating a particular birthday as a universal deadline or an age-associated risk as a certainty. What a father’s age may mean for a child’s health is covered in the next chapter.
Male fertility declines slowly and steadily with age, with no single cut-off; the woman’s age has the bigger effect.
Read more: Johnson et al. (2015): male age and semen quality, a review of 90 studies · Hassan & Killick (2003): male age and time to pregnancy · Begon et al. (2023): paternal age and live birth in donor-egg cycles · Murugesu et al. (2022): paternal age and fertility treatment outcomes · Nijs et al. (2011): male age in IVF and ICSI, adjusted for female age · WHO guideline for the prevention, diagnosis and treatment of infertility (2025)
Male age and your child’s health
Each year of a father’s age adds about one or two new genetic changes (de novo mutations) to his child’s DNA. A study that read the full genomes of 1,548 Icelandic families found about 1.5 extra changes from the father for each year of his age, against about 0.4 from the mother. Almost all of these changes are harmless, but they are the main suggested reason why older fathers have a slightly higher chance of some conditions.
A few rare conditions, such as achondroplasia (a form of short stature), arise mainly from new changes in the father’s sperm and become more likely with older fathers. They remain very rare.
Large population studies link older fatherhood with a modestly higher chance of autism and schizophrenia in children. In a study of about 5.7 million children in five countries, children of fathers aged 50 or older had about 1.66 times the risk of autism of children whose fathers were in their twenties. A review of schizophrenia studies found a similar relative risk for fathers aged 50 or older. These are relative risks: the great majority of children of older fathers do not have autism or schizophrenia, and the cause of the link is still debated.
Miscarriage risk also rises a little with the father’s age. A review of studies that took the mother’s age into account found about 1.2 times the risk when the father was 40 to 44, and about 1.4 times at 45 or older, compared with fathers aged 25 to 29. The mother’s age has a much bigger effect.
Most other links between a father’s age and his baby’s health, such as birth defects, stillbirth or being born early, are small. In a review of 238 studies, most pooled estimates were between 1.05 and 1.5 times, and none reached double. In about 40 million US births, babies of fathers aged 45 or older were slightly more likely to be born early.
Because of these findings, the 2024 US guideline on male infertility (AUA/ASRM) advises clinicians to tell couples in which the man is 40 or older about an increased risk of health problems for their children. This advice is based on expert opinion. It is information for planning, not a reason for alarm, and age is never the only factor.
Older fatherhood is linked with small increases in some risks for children, but the chance for any one child stays low.
Read more: Jónsson et al. (2017): new mutations in 1,548 Icelandic families · Kong et al. (2012): father’s age and new mutations · Goriely & Wilkie (2012): “paternal age effect” conditions · Sandin et al. (2016): parental age and autism in 5.7 million children · Miller et al. (2011): paternal age and schizophrenia, a review · du Fossé et al. (2020): paternal age and miscarriage, a review · Oldereid et al. (2018): fathers’ age and lifestyle and children’s health, a review · Khandwala et al. (2018): paternal age and 40 million US births · AUA/ASRM Male Infertility guideline, amended 2024
When to seek help, and what to bring
For couples having regular unprotected intercourse, evaluation usually begins after 12 months when the female partner is under 35, or after six months at 35 or older. From 40, more immediate evaluation may be appropriate. These thresholds should not postpone assessment when there is a known concern.
When couples usually ask for an assessment
- Female partner under 35After 12 months of regular unprotected intercourse.
- Female partner 35 to 39After six months.
- Female partner 40 or olderMore immediate evaluation may be appropriate.
- A known concernAsk sooner: an abnormal result, testicular history, erection or ejaculation difficulties, or treatment that may affect fertility.
Ask sooner with an abnormal semen result, a history affecting the testes, erection or ejaculation difficulties, repeated miscarriages, or treatment that may damage fertility. Both partners should be assessed together: European guidance recommends that every man in a couple having difficulty conceiving has a medical history and an examination, not only a semen test (a strong recommendation). This applies even if he has fathered a child before. A fertility appointment can also be a useful opportunity to discuss general health.
Bring previous results with dates, a medication and supplement list, details of recent fever or illness, and any relevant surgery or treatment history. Write down how long you have been trying and what you most want to understand.
Sudden severe testicular pain, testicular pain with nausea, vomiting or tummy pain, or pain that lasts more than an hour or continues when you rest, is an emergency: go to an emergency unit straight away, or call an ambulance on 10177 (112 from a cellphone), rather than waiting for a fertility appointment. A new lump, swelling or change in the shape of a testicle needs an urgent doctor’s appointment. Fertility information should never replace care for a new symptom.
When a couple is having difficulty conceiving, the man should be assessed alongside his partner, not after her.
Read more: ASRM: Fertility evaluation of infertile women: a committee opinion (2021) · EAU guideline: Male infertility · NHS: Testicle pain
What you may be wondering
Can I tell my sperm count from the amount of semen?
No. A semen analysis measures sperm concentration and other characteristics; the amount of fluid alone cannot answer that question.
When should I ask for an assessment?
Use the timing guidance above, and ask sooner with a known concern, an abnormal result or relevant treatment history. Both partners’ circumstances matter.
Does my age affect my child’s health?
Older fatherhood is linked with small increases in some risks, such as autism and miscarriage, but the chance for any one child stays low. The chapter on male age and your child’s health gives the numbers.
Common worries, explained carefully
“Semen and sperm are the same thing.”
Sperm are cells. Semen is the fluid that carries them, including secretions from reproductive glands. Fluid volume alone does not tell you the sperm count.
“Previous fatherhood means testing is unnecessary.”
Sperm health can change. Previous fatherhood is useful history, but it does not replace an assessment of the situation now.
“There is one birthday when male fertility stops.”
Age-related findings describe groups, not a deadline for an individual. Discuss age alongside your history and both partners’ assessment.
“A father’s age matters as much as a mother’s.”
The woman’s age has a much bigger effect on the chance of pregnancy and on miscarriage. A father’s age adds small increases in some risks.
The terminology, translated
- Spermatogenesis
- The process of making sperm in the testes.
- Testes
- Organs where sperm are produced and testosterone is made.
- Epididymis
- A coiled tube where sperm mature and are stored.
- Vas deferens
- A duct carrying sperm away from the epididymis.
- FSH
- Follicle-stimulating hormone: part of the hormone signalling that supports sperm production.
- LH
- Luteinising hormone: a signal that stimulates testosterone production in the testes.
- De novo mutation
- A new genetic change in a sperm or egg that neither parent carries in the rest of their body.
- Relative risk
- How many times more likely something is in one group than in another. On its own, it does not say how common it is.
- Could my medical history affect sperm production, transport or ejaculation?
- Do both partners need assessment now?
- Would hormone tests be useful in my situation?
- What symptoms or changes should I report promptly?
Videos on this guide
Videos on these questions are coming to the channel:
- Male Fertility 101
- Why Both Partners Need Fertility Testing
- Male Fertility: Questions Couples Often Forget to Ask
Sources & further reading
Sources 21 studies and guidelines Show the research
Prepared using Kara’s research library and checked against the linked clinical and patient-education sources. The explanations and new diagrams are original educational adaptations.
- WHO laboratory manual for human semen, sixth edition (2021)
- WHO guideline for the prevention, diagnosis and treatment of infertility (2025)
- Endotext: Endocrinology of the testis and spermatogenesis
- EAU guideline: Male infertility
- NICE NG257: Fertility problems: assessment and treatment (2026)
- AUA/ASRM Male Infertility guideline, amended 2024
- Johnson et al. (2015): male age and semen quality, a review of 90 studies
- Hassan & Killick (2003): male age and time to pregnancy
- Begon et al. (2023): paternal age and live birth in donor-egg cycles
- Murugesu et al. (2022): paternal age and fertility treatment outcomes
- Nijs et al. (2011): male age in IVF and ICSI, adjusted for female age
- Jónsson et al. (2017): new mutations in 1,548 Icelandic families
- Kong et al. (2012): father’s age and new mutations
- Goriely & Wilkie (2012): “paternal age effect” conditions
- Sandin et al. (2016): parental age and autism in 5.7 million children
- Miller et al. (2011): paternal age and schizophrenia, a review
- du Fossé et al. (2020): paternal age and miscarriage, a review
- Oldereid et al. (2018): fathers’ age and lifestyle and children’s health, a review
- Khandwala et al. (2018): paternal age and 40 million US births
- ASRM: Fertility evaluation of infertile women: a committee opinion (2021)
- NHS: Testicle pain
Fertility is a shared picture
For every couple, the health and results of both the man and the woman shape the plan. These Fertility Library topics sit alongside this guide.
