What a semen analysis measures
A standard semen analysis assesses several parameters together: the volume of the ejaculate, sperm concentration and total count, motility (the proportion of sperm moving, and how well), and morphology (the proportion with normal shape). No single number defines fertility; the parameters are interpreted as a set, and a low value in one area does not by itself mean a man cannot father a child.
The WHO 2021 reference values are derived from the lower fifth percentile of men who fathered a child within a year. They are reference points, not strict pass-or-fail thresholds, so results modestly below a reference value can still be compatible with conception.
Why one sample is rarely the whole story
Sperm parameters vary considerably between samples in the same man, influenced by abstinence interval, recent illness or fever, and even collection technique. For this reason an abnormal result is usually confirmed with a repeat test, often weeks apart, before drawing conclusions. Spermatogenesis takes roughly two to three months, so changes lag behind the events that caused them.
This is general educational information, not medical advice. Semen analysis results should be interpreted by a qualified clinician in the context of the couple.