In most cases it is due to a weakness of the pelvic floor muscles and is more common in women who have delivered children but also may be due to lack of exercise or simply getting older. Weakness of the pelvic floor muscles may be associated with a prolapse of the bladder into the vagina.
Leakage of urine may also be due to urge incontinence. It is due to an involuntary bladder contraction (detrusor instability) and its treatment is different from stress incontinence. There are other rare causes of incontinence, the doctor will assess you to exclude them.
