Invited commentary: monitoring fecundity over time--if we do it, then let's do it right.

A number of investigators have pointed to the possibility of a secular decline in human fecundity due to changes in sperm concentration. It is unlikely that any historical trends will be definitively quantified, but a good case can be made for more precise monitoring of this phenomenon in the future. Such monitoring would be justified on the grounds of the importance of early detection of environmental effects on the capacity of humans to reproduce. Establishing a surveillance system that will be sensitive enough to detect changes in fecundity over time is, however, a challenging enterprise because of methodological concerns. It may be impossible to obtain a quality of design that will pick up subtle changes in fecundity.