Many scholars have argued that deliberate birth spacing may have played a role before and during the modern fertility transition. There are good historical and theoretical reasons for this view, but it has proved to be hard to demonstrate convincingly that birth intervals were in fact partly determined by attempts at deliberate fertility control. This paper suggests a method of securing evidence on the issue for married couples. The method is applied to three cohorts living in a Belgian town in the nineteenth century. The findings indicate that, even before the fertility transition, couples in the working class were controlling their fertility by deliberate spacing.