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What causes UTIs & UI?

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UTIs in Women

A UTI develops when microbes (pronounced MAHY-krohbs) enter the urinary tract and cause infection. Bacteria are the most common cause of UTIs, although fungi rarely can also infect the urinary tract. E. coli bacteria, which live in the bowel, cause most UTIs.

The female anatomy contributes to women's increased likelihood of contracting a UTI.1 A woman's urethra is shorter than a man's, allowing bacteria better access to the bladder. A woman's urethral (pronounced yoo-REE-thruhl) opening is also close to sources of bacteria from the anus and vagina. Sexual activity can move bacteria to the urethral opening.

Having bacteria in the bladder does not always mean there is an infection. Like the bowel, the bladder has bacteria and other microorganisms that help to keep it healthy and functioning properly.

Some forms of birth control also increase the risk of UTIs. Spermicides can cause skin irritations that allow bacteria to invade. Diaphragms may slow urinary flow, encouraging bacteria to multiply. Unlubricated condoms or spermicidal condoms may cause irritation, which can help bacteria grow.1

The following factors also may encourage bacteria to grow:1

  • Not drinking enough fluids
  • Purposely holding in urine for long periods of time
  • Spinal cord injuries or other nerve damage that makes the bladder difficult to empty regularly and completely
  • Conditions or situations that block the flow of urine, such as a tumor, kidney stone, enlarged prostate, or sexual intercourse
  • Diabetes and other conditions that reduce the ability of the body's immune system to fight off infection
  • Catheters (tubes placed in the urethra and bladder to drain urine)
  • Hormonal changes in the urinary tract of pregnant women that make it easier for bacteria to spread through the ureters and to the kidneys

For information about UTIs in children, visit http://kidney.niddk.nih.gov/KUDiseases/pubs/utichildren/index.aspx.

UI in Women

UI is caused by problems with the muscles and nerves that hold or release urine. These muscles include bladder muscles, which contract to force urine into the urethra, and sphincter muscles that surround the urethra, which relax to allow urine to pass from the body. Incontinence occurs if bladder muscles suddenly contract or sphincter muscles are not strong enough to hold back urine.

These muscles also help to hold the urinary tract in place, so if the muscles are weakened, they may not be able to keep the bladder or other structures in the right position in the body. These types of structural problems, such as when the bladder is out of position, can also cause UI.

UTIs, vaginal infections or irritation, and medications can temporarily cause or aggravate UI. Constipation and being overweight or obese put pressure on the bladder and its controlling muscles and can also cause or aggravate UI.

Other features and conditions that can contribute to UI in women include:2

  • Weakened and stretched pelvic muscles after childbirth, with aging, or from genetic causes
  • Thinning and drying of the skin in the vagina or urethra after menopause
  • Weak bladder muscles or muscle spasms
  • Damage to nerves that control the bladder because of Parkinson's disease, multiple sclerosis, or physical injury
  • Age-related or behavioral changes
  • Diabetes
  • Obesity
  • Any injury, disability, or disease that makes it difficult to get to the bathroom promptly

For information about UI in men, visit http://kidney.niddk.nih.gov/KUDiseases/pubs/uimen/index.aspx.

For information about UI in children, visit http://kidney.niddk.nih.gov/KUDiseases/pubs/uichildren/index.aspx.


  1. National Institute of Diabetes and Digestive and Kidney Diseases, National Kidney and Urologic Diseases Clearinghouse. (2011). Urinary tract infections in adults. Retrieved May 15, 2012, from http://kidney.niddk.nih.gov/KUDiseases/pubs/utiadult/index.aspx [top]
  2. National Institute on Aging. (2011). Urinary incontinence. Retrieved May 16, 2012, from http://www.nia.nih.gov/health/publication/urinary-incontinence [top]

Last Updated Date: 11/30/2012
Last Reviewed Date: 04/12/2013
Vision National Institutes of Health Home BOND National Institues of Health Home Home Storz Lab: Section on Environmental Gene Regulation Home Machner Lab: Unit on Microbial Pathogenesis Home Division of Intramural Population Health Research Home Bonifacino Lab: Section on Intracellular Protein Trafficking Home Lilly Lab: Section on Gamete Development Home Lippincott-Schwartz Lab: Section on Organelle Biology