A kidney stone begins as a tiny piece of crystal in the kidney. When the urine leaves the kidney, it may carry the crystal out, or the crystal may stay in the kidney. If the crystal stays in the kidney, over time more small crystals join it and form a larger kidney stone.

Most stones leave the kidney and travel through the urinary tract when they are still small enough to pass easily out of the body. No treatment is necessary for these stones. But larger stones may become stuck in the tubes that carry urine from the kidney to the bladder (ureters). This can cause pain and possibly block the urine from flowing to the bladder and out of the body. The pain often becomes worse over 15 to 60 minutes until it is severe. The pain may ease when the stone no longer blocks the flow of urine, and it often goes away when the stone passes into the bladder. Medical treatment is often necessary for larger stones.

• The smaller a stone is, the more likely it is to exit the body (pass) on its own. About 9 out of every 10 stones smaller than 5 mm and about 5 out of every 10 stones 5 mm to 10 mm pass on their own.¹ Only 1 or 2 out of every 10 kidney stones need more than home treatment.¹
• The average time a stone takes to pass ranges between 1 and 3 weeks,¹ and two-thirds of stones that pass on their own pass within 4 weeks of when the symptoms appeared.²
• Almost half of all people who get kidney stones will get more stones within 5 years unless they take preventive measures.³ When you have kidney stones several times over a period of years, the length of time between stones tends to get shorter. It is not possible to predict who will have more stones in the future and who will not.

Problems that may occur with kidney stones include:

• An increased risk of urinary tract infection, or making an existing urinary tract infection worse.
• Kidney damage, if stones block the flow of urine out of both kidneys (or out of one kidney, for people who have a single kidney). For most people with healthy kidneys, kidney stones do not cause serious damage until they completely block the urinary tract for 2 weeks or longer.⁴

Kidney stones are more serious for people who have a single kidney or an impaired immune system or have had a kidney transplant.

Stones in pregnant women

When stones occur during pregnancy, an obstetrician and urologist should determine whether you need treatment. Treatment will depend on your trimester of pregnancy.⁵

Stones in children

Kidney stones are not common in children. When they occur, it is usually between the ages of 8 and 10. Children with kidney stones may also have a urinary tract infection. Often, children with stones also have other medical problems, such as an abnormally developed urinary system, a metabolic disorder, or genetic risks, such as cystic fibrosis.⁶