Male fertility and sperm health: what the numbers mean and what changes them

Roughly half of couples who struggle to conceive have a male factor involved, and the first test is a semen analysis. It is also the test most likely to be misread — by patients, and by websites quoting figures that changed five years ago.

Two things are worth establishing before anything else. The reference values in current UK use are not the ones most search results give. And whatever you change, the result will not move for about three months.

The reference values, correctly

UK practice follows NICE NG257, published on 31 March 2026 and replacing the 2013 guideline CG156 ^[1]. The reference values come from the WHO laboratory manual, 6th edition (2021) ^[2]:

A great deal of UK patient material, and most of what circulates online, still quotes the WHO 5th edition (2010) figures — 15 million/ml, 32% progressive motility, 1.5 ml volume. Those numbers are not catastrophically different, but if you are comparing your result against a website you may be comparing it against the wrong line.

What the numbers are not

This matters more than the values themselves.

These are fifth-centile lower reference limits, derived from men whose partners conceived within twelve months. By construction, one in twenty men who successfully fathered a child falls below them. They are not a pass mark, they are not a threshold, and a result below a limit is not a diagnosis of infertility.

They also are not beyond criticism. The statistical basis of the 6th edition reference population has itself been challenged in the peer-reviewed literature ^[3]. A semen analysis is one piece of information about a couple, interpreted alongside the female partner’s assessment and how long they have been trying.

Repeat testing

One sample is not a result. Values fluctuate considerably between samples from the same man.

NG257 advises repeating the analysis ideally three months after the first, so the second sample reflects a complete cycle of sperm production, and sooner where there is gross deficiency ^[1]. Typical UK laboratory practice repeats within two to four weeks if total sperm number is below 7 million or the sample appears to contain no sperm at all, and at three months where progressive motility is under 10%, morphology is 0–1%, or very few sperm are present.

Why everything takes three months

Spermatogenesis — the production of a sperm cell from its precursor — takes approximately 74 days. Add roughly two weeks of maturation and transit through the epididymis, and the sperm in today’s sample began forming around three months ago.

Every practical consequence follows from this. Stopping smoking, losing weight, stopping steroids, changing a medicine: none of it will show in a sample taken four weeks later, because those sperm were already made. Anyone offering a faster result is not describing human biology.

It also means the three months are not wasted. They are the window in which changes are actually taking effect.

What the guideline says about lifestyle

NG257 is careful about what is established and what is not, and it is worth reproducing that care rather than overclaiming ^[1].

Alcohol. Excessive alcohol damages semen quality. Drinking within the UK Chief Medical Officers’ guideline — no more than 14 units a week, spread over three or more days — is described as unlikely to affect semen quality ^{[1, 6]}. Note the UK framing in units; American advice in “standard drinks” is not the same measure.

Smoking. Associated with reduced semen quality, though the effect on actual fertility is uncertain. Support to stop is recommended regardless ^[1].

Weight. A BMI of 30 or above is associated with increased risk of reduced fertility ^[1]. Our guide to weight and fertility covers the mechanisms in both partners.

Scrotal temperature. Elevated scrotal temperature is associated with reduced semen quality — but the guideline explicitly states it is uncertain whether wearing loose-fitting underwear improves fertility ^[1]. This is worth saying plainly, because the internet is confident about laptops, hot baths and boxer shorts in a way the evidence is not.

Occupation. Worth asking about hazards — heat, solvents, pesticides, heavy metals.

Recreational drugs. NG257 advises asking specifically about anabolic steroids and cannabis ^[1].

The medicines to ask about

The 2026 update names three explicitly, and one of them is new ^[1].

Testosterone replacement. This suppresses the hormonal axis that drives sperm production, collapsing the testosterone concentration inside the testis and producing very low or absent sperm counts. It should not be given to a man seeking conception. Recovery after stopping, from pooled trial data, reaches 20 million/ml in around 67% by six months, 90% by twelve months and close to 100% by 24 months ^[4]. Our guide to testosterone replacement in the UK covers this in more detail.

Finasteride. Reduces ejaculate volume. Effects on sperm count at the lower dose used for hair loss are inconsistent and generally small, though case reports describe marked reductions that reversed on stopping. Being named in NG257 raises the bar for counselling men who are trying to conceive. Our guide to finasteride side effects covers the wider profile.

GLP-1 receptor agonists. Newly named in the 2026 guideline. The evidence here is preliminary, and weight loss itself generally improves semen parameters, so this should be read as an area of active uncertainty rather than established harm.

Others worth raising with a prescriber include sulfasalazine, chemotherapy agents, ketoconazole, spironolactone and long-term opioids. Do not stop a prescribed medicine on the strength of this list — raise it with your prescriber, who can weigh it against why you are taking it.

Anabolic steroids

The single most important reversible cause in young men who turn out to have no sperm in a sample, and the one most often not volunteered.

The mechanism is the same as with testosterone replacement: external androgens suppress the pituitary signals that drive the testes. Recovery is slower in this group — mean around 10.4 months, with FSH normalising at around 19 months and inhibin B at around 31 months ^[4]. Longer use, higher doses, older age and pre-existing testicular problems all slow it further.

Salvage treatments exist and are used in specialist andrology practice, but they are off-label in the UK and specialist-initiated. Our guide to hair loss and anabolic steroids covers the other visible consequences.

Telling a clinician honestly changes the tests and the advice. It is worth doing.

When to seek help

If you have been trying for 12 months without conception, both partners should be assessed. Sooner — around six months — if the female partner is over 36, or if there is a known reason for concern on either side.

Any testicular lump, hard or irregular testis, or testicular swelling needs an urgent GP appointment on the suspected cancer pathway, regardless of fertility. Our guide to when to see someone about a sexual problem sets out the other red flags.

This article is for general information and isn’t a substitute for advice from your own clinician. If you feel unwell, contact your GP or NHS 111. In an emergency, call 999.