The purpose of this study was to estimate the frequency of post-caesarean section scar defects (PCSD) on transvaginal ultrasound and to correlate these with the presence of abnormal menstrual bleeding. We reviewed the pelvic ultrasound images of 318 women referred for a gynaecological scan in a private women's ultrasound clinic. The incidence of defects at the expected location of a caesarean scar was evaluated and then correlated with patient responses to a questionnaire on abnormal menstrual bleeding. Of the 318 patients, 71 had a history of caesarean section and technically adequate pelvic ultrasound images. Fluid-filled defects in the hysterotomy incision were noted in 29 (40%). The presence of a PCSD was significantly associated with patients reporting prolonged periods or post-menstrual spotting. The larger the defect, the higher was the incidence of abnormal vaginal bleeding.