Declining sperm quality worldwide: what the studies show

What the landmark meta-analysis reports

The meta-analysis by Levine and colleagues, published in Human Reproduction Update, pools 223 studies from 53 countries covering data from 1973 to 2018. It reports a global decline of 51.6% in sperm concentration and of 62.3% in total sperm count per ejaculate, with a steeper slope since the 2000s.

It is the most cited work on the topic, and the one that brought the question into public debate.

The scientific debate, presented honestly

These conclusions are contested by part of the scientific community. The main criticisms concern methodology: over-representation of European and North American data in the older series, heterogeneous counting methods between laboratories and across decades, and selection of the populations studied (donors, infertility patients, military recruits).

These criticisms do not demonstrate the absence of a decline; they call for caution about its exact size. The area of consensus is narrower: a downward trend is likely, its precise quantification remains debated.

Suggested causes, none of them confirmed

No single cause explains this trend. The most discussed hypotheses are:

• smoking,
• excess weight and metabolic syndrome,
• chronic stress and sedentary behaviour,
• air pollution,
• exposure to endocrine disruptors,
• occupational exposures, notably to pesticides and solvents,
• heat exposure.

The essential individual nuance

A population trend does not translate mechanically to the individual level. World Health Organization reference values place normal concentration in a wide range, of the order of 15 to 200 million sperm per millilitre: a concentration lower than thirty years ago can remain perfectly compatible with spontaneous conception.

Only a semen analysis, interpreted by a professional and usually repeated, can situate an individual case.