Aim: To study changes in a ceasarean section (CS) scar during the first year after a CS using gel installation sonography (GIS). Methods: Proof-of-concept study, prospective cohort study. Twenty women who delivered by their first CS were evaluated by both transvaginal sonography and GIS 2 months and 1 year after CS. A niche was defined as an anechogenic space at the uterine caesarean scar with a depth >2 mm. The primary outcome was any change in the thickness of the residual myometrium (RMT) as evaluated by GIS. Results: Mean RMT changed in time from 11.9 mm at 2 months to 6.5 mm at 12 months after the CS (p < 0.001). Niche prevalence did not change. The adjacent myometrium (AM) reduced from 15 to 12.4 mm (p = 0.04). The ratio between RMT and AM with GIS decreased from 0.80 at 2 months to 0.54 at 12 months (p = 0.002). Conclusion: RMT thickness, the adjacent myometium and the ratio between the RMT and AM reduces from 2 to 12 months after a CS. The prevalence did not change. This needs to be taken into account when deciding on the timing of niche measurement and the interpretation of the RMT.