It has been demonstrated for many pre-industrial populations that the age at marriage, or marriage duration, influences age-specific marital fertility but the reason for this remains unclear. Among the several mechanisms that may be responsible, the following are often cited: secondary sterility or increased subfecundity associated with parity; declining coital frequency; the age difference between the spouses; and, importantly, parity-dependent fertility control. If the latter mechanism were partly responsible for the marriage-duration effect in pre-transition populations, it would contradict the concept of the modern fertility transition as the evolution (or revolution) from parity-independent to parity-dependent fertility. The study presented in this paper investigates the relative importance of these alternative explanations. The application of multivariate Poisson regression to the fertility data from two birth cohorts in the Belgian city of Leuven shows that a linearly declining or even concave age-specific fertility pattern, disaggregated by age at marriage, does not imply parity-dependent fertility limitation.