What a semen analysis can and cannot tell a man about his fertility

A semen analysis is the first and central test when a couple cannot conceive, and for most men it is the only fertility test they will ever have [s1]. But it measures whether a sample's sperm numbers sit above or below reference limits derived from men who are already known to be fertile — not whether a given man can or cannot father a child [s2][s3]. That distinction is the whole of what the test can honestly deliver, and it is routinely lost.

What the numbers are

The reference values in wide use come from the World Health Organization's laboratory manual, whose sixth edition was launched in 2021 and can be downloaded free [s2]. Its "lower reference limits" are the fifth centile of results from fertile men — the value below which only the least fertile 5% of that reference group fell [s2]. In the 2021 edition those limits are a semen volume of 1.4 mL, a total sperm number of 39 million per ejaculate, total motility of 42%, progressive motility of 30%, vitality of 54%, and 4% normally-formed sperm [s2].

Most of these barely moved from the fifth edition of 2010, which set volume at 1.5 mL, total motility at 40%, progressive motility at 32%, vitality at 58% and normal forms at the same 4% [s2]. The total sperm number was unchanged at 39 million [s2]. These are not large revisions, and none of them redraws the line between fertile and infertile in a way a man would feel.

Where the numbers come from — and why they wobble

The sixth edition's reference population was 3,589 fertile men, drawn from 10 studies across 12 countries and five continents [s3]. That is a broader base than earlier editions, but a 2022 reappraisal in Human Reproduction found it does not behave like a single population [s3]. When the analysts checked how well the pooled centiles described each contributing study, 88 of 117 ranges — 75% — differed significantly from what the reference distribution predicted, a sign that laboratory methods and the men themselves varied too much to be merged into one yardstick [s3]. The fifth centile of total sperm count was the only cut-off consistent across most studies [s3].

The practical lesson is that a semen result carries real measurement noise, from the laboratory as much as from the man. A count that varies from one sample to the next is expected, which is why a single low reading is not treated as a verdict [s3].

What a result actually means

Because the limits are simply where fertile men's results thinned out, they discriminate poorly between fertile and infertile individuals [s3]. A man can sit below every threshold and conceive naturally; another can clear all of them and still be part of an infertile couple, because the female partner's evaluation and the couple's history matter at least as much [s1][s3]. The reappraisal's authors put the concern plainly — the reference populations are a weak tool for telling fertile men from infertile ones [s3]. A semen analysis narrows the possibilities and flags the extremes, such as the absence of sperm; it does not issue a probability that this couple will conceive.

That is also why the American Urological Association and American Society for Reproductive Medicine treat the analysis as the opening move of an evaluation rather than its conclusion, set alongside a reproductive history and physical examination, with further testing where those point somewhere specific [s1]. The same guideline makes a point too often skipped: detecting male infertility is associated with a raised risk of other health problems later, so an abnormal result is a reason to look at the man's broader health, not only his sperm [s1].

None of this makes the test optional — it is the necessary first step, and the honest reading of a low number is "look further and repeat," not "sterile." A specific cause such as a varicocele may be found and treated; a reversible one, such as testosterone therapy suppressing sperm production, may be identified and stopped. It should also temper the headlines about a population-wide decline in sperm counts, which lean on the same noisy measurement, and it sits within the wider, mostly modifiable set of male-fertility risk factors that a workup is meant to surface.