Risk factors are elements that increase a person’s chance of developing a disease or condition. However, not everyone with one or more risk factors will develop the condition. It is also possible that people with no risk factors will develop the condition. Those factors known to increase a woman’s risk of endometriosis include:

• Not giving birth (nulliparity). Women who have not given birth and have not had their menstrual cycles interrupted by pregnancy are at greater risk.
  
  
• Family history. Women with a mother or sister with endometriosis have a higher risk of developing the condition.
  
  
• Length of menstrual cycle. Women with menstrual cycles shorter than 27 days or whose periods last longer than eight days are at greater risk.
  
  
• Age. The risk for endometriosis begins with the onset of menstruation and increases with age, until menopause.
  

Physicians do not know what causes endometriosis. Several theories have been proposed and are still being studied, including:

• Retrograde menstruation. In normal menstrual flow, the endometrial tissue breaks up in the uterus and exits through the vagina. Many women experience some retrograde (backward) menstruation, where blood flows in reverse to the fallopian tubes. This misplaced blood and endometrial tissue may be a cause of endometriosis. However, most women normally have some retrograde menstrual flow and do not develop endometriosis.
  
  
• Blood and lymphatic transportation. Endometrial cells may be transported away from the uterus in the circulatory or lymphatic systems. The cells are deposited at distant sites and develop in response to the hormonal cycles.
  
  
• Metaplasia. The ability of one type of cell to develop into another type. For example, cells of the peritoneum (pelvic lining) may change into endometrial cells. This change may be stimulated by a malfunction in the immune system or irritation from endometrial tissue that retrograded in menstruation. 
  
  
• Immune system changes. Normally, the immune system should fight endometrial tissue that is located outside the uterus. An impaired immune system may allow endometriosis to develop, either from retrograde menstrual flow or from endometrial implants that develop from other cells.
  
  
• Environmental causes. Some researchers are studying the effects of certain chemicals such as dioxins as potential causes of endometriosis.