Sometimes, adenomyosis causes no signs or symptoms or only mild discomfort. However, adenomyosis can cause:

• Heavy or prolonged menstrual bleeding
• Severe cramping or sharp, knifelike pelvic pain during menstruation (dysmenorrhea)
• Chronic pelvic pain

Your uterus might get bigger. Although you might not know if your uterus is enlarged, you may notice that your lower abdomen feels tender or causes pelvic pressure.

The cause of adenomyosis isn't known. There have been many theories, including:

• Invasive tissue growth. Some experts believe that adenomyosis results from the direct invasion of endometrial cells from the lining of the uterus into the muscle that forms the uterine walls. Uterine incisions made during an operation such as a cesarean section (C-section) might promote the direct invasion of the endometrial cells into the wall of the uterus.
• Developmental origins. Other experts suspect that adenomyosis originates within the uterine muscle from endometrial tissue deposited there when the uterus first formed in the fetus.
• Uterine inflammation related to childbirth. Another theory suggests a link between adenomyosis and childbirth. Inflammation of the uterine lining during the postpartum period might cause a break in the normal boundary of cells that line the uterus. Surgical procedures on the uterus can have a similar effect.
• Stem cell origins. A recent theory proposes that bone marrow stem cells might invade the uterine muscle, causing adenomyosis.

Regardless of how adenomyosis develops, its growth depends on the circulating estrogen in women's bodies.

Risk factors for adenomyosis include:

• Prior uterine surgery, such as a C-section or fibroid removal
• Childbirth
• Middle age

Most cases of adenomyosis — which depends on estrogen — are found in women in their 40s and 50s. Adenomyosis in these women could relate to longer exposure to estrogen compared with that of younger women. However, current research suggests that the condition might be common in younger women.

If you often have prolonged, heavy bleeding during your periods, you can develop chronic anemia, which causes fatigue and other health problems.

Although not harmful, the pain and excessive bleeding associated with adenomyosis can disrupt your lifestyle. You might avoid activities you've enjoyed in the past because you're in pain or you worry you might start bleeding.