A normal uterus is made up of the endometrium on the inside, the myometrium (muscle layer) in the middle, and the serosa on the outside. In adenomyosis, endometrial tissue penetrates deep into the myometrium and repeatedly bleeds and inflames each month like menstruation, causing the uterus to enlarge overall. This differs in location from endometriosis, where lesions form outside the uterus on the ovaries, peritoneum, and elsewhere. It is also common for both conditions to occur together. Adenomyosis is most often diagnosed in women in their 30s and 40s who have given birth, and it is confirmed with ultrasound or MRI by identifying uterine enlargement and a non-uniform myometrium.