Is mild hydronephrosis (kidney swelling due to urine accumulation) on ultrasound in a first trimester female a normal finding?

From the Guidelines

Mild hydronephrosis on ultrasound in a first trimester female is generally considered normal due to physiological changes during pregnancy. This condition is a result of mechanical obstruction from an enlarged uterus and collecting system smooth muscle relaxation caused by progesterone, as noted in the study by 1. The study highlights that asymptomatic hydronephrosis occurs in an estimated 70% to 90% of pregnant patients, typically asymmetrically prominent on the right. Key points to consider include:

• The primary cause is mechanical compression of the ureters by the enlarging uterus, particularly on the right side, along with hormonal effects from progesterone which relaxes smooth muscle in the urinary tract 1.
• This physiological hydronephrosis usually resolves spontaneously, and its prevalence is higher in advancing trimesters and in multiparous patients.
• Approximately 0.2% to 4.7% of pregnant patients experience symptomatic hydronephrosis, which may lead to preterm labor or maternal and/or fetal death when left untreated 1. It's essential to distinguish physiological hydronephrosis from pathological causes such as urinary tract infections or kidney stones, which would require appropriate treatment. If the pregnant woman experiences fever, significant pain, or other concerning symptoms, further evaluation is warranted to rule out complications. In the absence of symptoms or complications, no specific treatment is needed for mild physiological hydronephrosis in pregnancy, as supported by the most recent evidence from 1.

From the Research

Definition and Diagnosis of Hydronephrosis

• Hydronephrosis is diagnosed using various modalities, including sonography, radiography, magnetic resonance imaging, and nuclear medicine technology 2.
• Ultrasound remains the cornerstone of initial investigation, with MRI in reserve if the diagnosis remains uncertain 3.

Hydronephrosis in Pregnancy

• Approximately 3% of pregnancies are complicated by symptomatic hydronephrosis and up to 0.8% with urolithiasis 3.
• Congenital hydronephrosis is frequently amenable to prenatal diagnosis, often as early as the second trimester 4.
• Most clinicians use a renal pelvis anterior-posterior (AP) diameter of 4 mm or more prior to 20 weeks of gestation as a threshold for identifying pyelectasis 4.

Mild Hydronephrosis on Ultrasound in First Trimester

• If mild dilation of the renal pelvis is an isolated finding in the second trimester, evaluation performed later in gestation is used to guide postnatal management 4.
• Since the normal renal pelvis dimensions may increase with advancing gestation, thresholds for the diagnosis are larger in the third trimester 4.
• There is no direct evidence to suggest that mild hydronephrosis on ultrasound in a first trimester female is abnormal, but it is essential to consider the gestational age and renal pelvis diameter when making a diagnosis 4.

Management of Hydronephrosis

• Conservative approaches for symptomatic hydronephrosis as well as ureteral calculi is the preferred initial management option in the well patient 3.
• If intervention is required, ureteral stent and nephrostomy can be used with informed consent on the risks and benefits of each 3.
• The role of doppler ultrasonography in managing hydronephrosis during pregnancy can help determine the Resistive Index (RI) and the difference between the RI of the corresponding and contralateral kidney (Delta Resistive Index = delta RI) 5.