This article examines the factors that influence the formation and development of fertility plans over a lifetime. In particular, the hypothesis is made that the desired number of children a person reports reflects their experience of family life. On the basis of data from the Intentions de fécondité survey carried out by INED and INSEE in France in 1998, and qualitative interviews with young parents, it is observed that people who have not yet had a child form their plans in terms of the only family references they have, namely their own childhood and notably their own sibship size (the more brothers and sisters they had, the more children they want, on average), but this effect is not found among the parents of one or two children. The experience of having a first child brings home the practical implications of parenthood (increasing difficulties in reconciling family and job, for example), and so the desired number of children corresponds to a different way of thinking. However, the results of the EHF family history study (INSEE 1999) show that own sibship size does ultimately have an effect on completed fertility, which casts some doubt on the validity of questions about intentions.