The pelvis of the kidney (renal pelvis), not to be confused with the pelvic bones at the bottom of the spine, is the central area of the kidney where urine collects briefly before it freely flows into the ureter, the muscular tube that pumps urine down into the bladder.
Normally, urine is taken immediately into the ureter so that the pelvis of the kidney is only a small space. This area is routinely measured by ultrasound examinations done in pregnancy. When this area is found larger than usual, based on what has been measured in thousands of other normal pregnancies, it is called fetal pelviectasis or fetal pyelectasis. When the space is huge and ballooned, it is called fetal hydronephrosis.
When the urine is unable to flow freely into the ureter due to a blockage (obstruction), such as a kink in the ureter or a narrowing of where the ureter meets the renal pelvis, (the so-called “ureteropelvic junction”), is a condition named ureteropelvic junction obstruction, or UPJO. UPJO usually causes problems to the kidney in the ways explained above, and for that reason, it is usually surgically corrected after birth, sometimes as an emergency.