Pyelectasis is a common finding during pregnancy that refers to the dilation of the renal pelvis, the funnel-shaped structure in the kidney that collects urine before it flows into the ureter. It is typically detected during routine prenatal ultrasound screening. While it is usually temporary and resolves on its own, it can occasionally signal an underlying urinary tract issue. The condition requires careful monitoring both before and after birth to ensure the health of the developing kidney.
The Anatomy and Grading of Pyelectasis
The renal pelvis is the central part of the kidney that serves as the main collection point for urine. When the anteroposterior diameter (APD) of this pelvis appears enlarged on an ultrasound scan, it is diagnosed as pyelectasis or renal pelvic dilation. This dilation can occur because of a temporary back-up of urine or an immature urinary tract system.
Physicians use specific millimeter measurements to grade the severity of the pyelectasis, which helps determine the follow-up protocol. In the second trimester (before 28 weeks), a measurement of 4 to 7 millimeters is classified as mild pyelectasis, while 7 to 10 millimeters is considered moderate. After 28 weeks, the criteria shift slightly, with mild often being 7 to 10 millimeters.
Any measurement greater than 10 millimeters, regardless of the gestational age, is classified as severe pyelectasis, sometimes referred to as hydronephrosis. The severity grading helps medical teams assess the likelihood of a postnatal problem. Mild cases have a high rate of spontaneous resolution.
Detection and Monitoring During Pregnancy
Pyelectasis is most frequently discovered during the standard anatomy scan, performed around the 20th week of pregnancy. This routine ultrasound allows the technician to measure the renal pelvis and identify any enlargement. The finding is more common in male fetuses and can affect one or both kidneys.
The protocol for monitoring pyelectasis involves serial follow-up ultrasounds to track the size of the renal pelvis. The purpose of this close surveillance is to determine if the dilation remains stable, decreases, or progresses over time. For mild cases, a repeat scan in the third trimester, around 32 weeks, is often recommended to check for persistence or worsening.
If the pyelectasis is isolated, meaning no other fetal anomalies are present, the overall prognosis is generally excellent. Prenatal monitoring allows the medical team to prepare for the appropriate postnatal evaluation if the dilation persists or increases. A significant portion of mild pyelectasis cases, up to 80%, resolve spontaneously before birth.
Postnatal Outcomes and Clinical Management
For many infants, pyelectasis resolves either before birth or within the first year of life without intervention. However, because persistent dilation can signal an underlying issue like a blockage or the backward flow of urine, postnatal evaluation is required. The initial step is usually a renal and bladder ultrasound, performed within the first few days or weeks after birth, which provides a clearer image of the urinary tract.
If the postnatal ultrasound shows persistent or moderate-to-severe dilation, further testing may be necessary to identify the cause. A common concern is vesicoureteral reflux (VUR), where urine flows backward from the bladder toward the kidneys.
Diagnostic Testing
VUR is diagnosed using a voiding cystourethrogram (VCUG). This test involves placing a catheter into the bladder and injecting a contrast agent to observe the flow of urine during voiding.
In some cases, particularly those with higher-grade dilation, prophylactic antibiotics may be prescribed to prevent urinary tract infections (UTIs) until a definitive diagnosis is made. Prevention of UTIs is a priority because these infections can lead to kidney damage and scarring in infants. Most mild pyelectasis cases that do not resolve are managed conservatively with continued observation through serial ultrasounds.
Only a small percentage of children with prenatally detected pyelectasis, typically less than 6%, will ultimately require surgical intervention. This is generally reserved for cases where a confirmed obstruction, such as a ureteropelvic junction obstruction, is causing a significant risk to kidney function. Referral to a pediatric urologist is standard practice for infants with persistent moderate or severe dilation.