With the increasing interest in population dynamics, research workers with differing backgrounds, such as demographers, vital statisticians and members of the medical profession, find themselves on common ground. However, this interdisciplinary contact reveals that we sometimes use technical words in different ways, depending on the terminology of our profession. When a doctor speaks about fertility, it is synonymous with the Scandinavian word frugtbarhed whereas fertilitet for the demographer only refers to live-born. Medical research within the field of fertility usually deals with conceptions, i.e. live-births, still-births, legal abortions and other hospitalized abortions, including illegal and spontaneous abortions. There are also an unknown number of conceptions, such as non-hospitalized abortions, but it is now generally believed that, in Denmark, this figure is negligible (Hoffmeyer & Norgaard, 1964; Sele & Norgaard, 1972). Moreover, during the last three decades, contraception has become more and more widespread and this new variable, which influences both births and abortions, should also be considered. Bearing this in mind it would be appropriate to refine our terminology and use ‘manifest fertility’ for conceptions and ‘potential fertility’ for all biologically possible pregnancies.