BACKGROUND Studies on fertility preferences have highlighted a gap between preferred and realised fertility. Here we examine two potential determinants of this fertility gap: how certain people are in their preferred fertility, and to what extent there is partner agreement on this preferred number. METHODS A sample of 758 women between the ages of 18 and 40 were recruited through the Longitudinal Internet Studies for the Social Sciences-panel, a representative sample of Dutch individuals.RESULTS Only 21% of women without children and 48% of mothers were certain about their fertility preferences. With increasing preferred number of children, certainty in this preference decreased. More than half of respondents agreed with their partner on the preferred number, and agreement decreased for increasing preferred number of children. There was agreement in 90% of the couples on not having children, and 80% of mothers agreed with their partner on having only one child. This decreased to 40% agreement in women that preferred more than three children. CONCLUSIONS Dutch women were uncertain about their fertility preferences, particularly in those without children. There was more uncertainty and partner disagreement surrounding preferences for larger than for smaller number of children. Both uncertainty and disagreement likely lower fertility. CONTRIBUTION This study gives a detailed look on how the certainty of fertility preferences and partner agreement vary by the preferred number of children. Findings help explain the persistent gap between preferred fertility and realised fertility.