Fertility is a couple concept that has been measured since the beginning of demography, and male fecundity (his biological capacity
to reproduce) is a component of the fertility rate. Unfortunately, we have no way of measuring the male component directly, although
several indirect markers can be used. Population registers can be used to monitor the proportion of childless couples, couples
who receive donor semen, trends in dizygotic twinning, and infertility diagnoses. Studies using time-to-pregnancy (TTP) may
identify couple subfecundity, and TTP data will correlate with sperm quality and quantity as well as sexual activity and a number
of other conditions. Having exposure data available for couples with a fecund female partner would make TTP studies of interest
in identifying exposures that may affect male fecundity. Biological indicators such as sperm quality and quantity isolate the male
component of fertility, and semen data therefore remain an important source of information for research. Unfortunately, often over
half of those invited to provide a sperm sample will refuse, and the study is then subject to a selection that may introduce bias.
Because the most important time windows for exposures that impair semen production could be early fetal life, puberty, and the
time of ejaculation; longitudinal data over decades of time are required. The ongoing monitoring of semen quality and quantity
should continue, and surveys monitoring fertility and waiting TTP should also be designed.

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