Posttraumatic reactions are common among mothers of preterm infants and can have a negative influence on their quality of life and lead to interactional difficulties with their baby. Given the possible trajectories of posttraumatic reactions, we hypothesized that prevalences of postpartum posttraumatic reactions at given times underestimate the real amount of mothers experiencing these symptoms within 18 months following delivery. Additionally, we examined whether sociodemographic and clinical characteristics of dyads influence the expression of posttraumatic symptoms among these mothers. A sample of 100 dyads was included in this longitudinal study led by 3 french university hospitals. Preterm infants born before 32 weeks of gestation and their mothers were followed-up over 18 months and attended 5 visits assessing the infants' health conditions and the mothers' psychological state with validated scales. Fifty dyads were retained through the 18 months of the study. The period prevalence of posttraumatic reactions was calculated and a group comparison was conducted to determine their predictive factors. Thirty-six percent of the mothers currently suffered from posttraumatic symptoms 18 months after their preterm delivery. The 18 months period prevalence was 60.4% among all the mothers who participated until the end of the follow-up. There was a statistical link between posttraumatic symptoms and a shorter gestational age at delivery, C-section, and the mother's psychological state of mind at every assessment time. Only a small proportion of mothers were receiving psychological support at 18 months. Preterm mothers are a population at risk of developing a long-lasting postpartum posttraumatic disorder, therefore immediate and delayed systematic screenings for posttraumatic symptoms are strongly recommended to guide at-risk mothers towards appropriate psychological support.