Pregnancy changes the urinary tract. The kidneys filter more blood, the hormone progesterone relaxes the ureters, and the enlarging uterus can compress them. As a result, the kidney drainage system and ureters often become dilated, a finding called hydronephrosis or hydroureter.

In most women this is a normal, temporary effect of pregnancy. However, hydronephrosis can also be caused by a kidney or ureteric stone, infection or, less commonly, another obstruction. The challenge is deciding whether the dilatation is physiological or whether the kidney is genuinely blocked and needs treatment.

Seek urgent medical assessment for flank pain accompanied by fever, chills, feeling faint or very unwell, persistent vomiting, difficulty passing urine, reduced fetal movement, contractions, vaginal bleeding or fluid loss. An infected, obstructed kidney is a urological emergency.

Why does hydronephrosis occur during pregnancy?

Physiological hydronephrosis usually develops during the second trimester, becomes most marked around 24–28 weeks and is often greater on the right. It results from:

• progesterone-related relaxation of ureteric smooth muscle;
• compression of the ureters by the growing uterus at the pelvic brim;
• rotation of the uterus, which tends to place more pressure on the right ureter; and
• increased urine production during pregnancy.

This dilatation commonly settles within several weeks after delivery. Physiological hydronephrosis is not itself a stone and does not automatically require a stent or operation.

Features that make pathological obstruction more likely include severe colicky flank pain, blood in the urine, a visible stone, dilatation extending below the pelvic brim, an absent or reduced ureteric jet on the symptomatic side, impaired kidney function, infection or worsening hydronephrosis. No single ultrasound sign is perfect; the symptoms, blood and urine tests, imaging and obstetric assessment must be considered together.

Kidney and ureteric calculi in pregnancy

Urinary calculi are stones located in the kidney or ureter. They are an important cause of non-obstetric abdominal or flank pain during pregnancy and occur most often in the second or third trimester. Typical symptoms include:

• sudden pain in the loin, flank, abdomen or groin, which may come in waves;
• nausea and vomiting;
• visible or microscopic blood in the urine;
• urinary frequency, urgency or burning; and
• fever or rigors if infection is present.

Pregnancy increases urinary calcium and urate excretion, while other natural protective factors also rise. For this reason, pregnancy does not necessarily increase the overall number of stones, although the stone pattern may differ and calcium-phosphate stones are relatively more common.

Why infection matters

Pregnancy-related ureteric dilatation and slower urinary drainage encourage urinary stasis. A stone can worsen this by partly or completely blocking the ureter. Bacteria trapped above an obstruction may cause pyelonephritis, pyonephrosis, bacteraemia or sepsis.

All women with suspected renal colic should therefore have urinalysis and a urine culture. Blood tests commonly include a full blood count, kidney function, electrolytes and inflammatory markers. Antibiotics are selected according to pregnancy safety, local resistance patterns and culture results.

Antibiotics alone are not sufficient when infection is trapped behind an obstructed kidney. Urgent decompression with a ureteric stent or percutaneous nephrostomy is required, together with intravenous antibiotics and coordinated obstetric care. Definitive stone treatment is usually delayed until sepsis has resolved.

Is there a risk of premature labour?

Observational studies associate symptomatic stones and renal obstruction in pregnancy with higher rates of urinary infection, hospital admission, preterm contractions and preterm delivery. Severe pain, dehydration, inflammation and infection may all contribute. However, the absolute risk for an individual woman varies, and an association does not mean that every renal colic episode will trigger premature labour.

The condition requiring intervention may itself be responsible for some of the reported risk. For this reason, treatment decisions should not be based on procedure statistics alone. The balance is between allowing a stable stone time to pass and promptly treating uncontrolled pain, infection or threatened kidney function. Depending on gestational age and clinical circumstances, fetal monitoring and assessment for contractions may be appropriate.

How is it investigated?

1. Ultrasound first

Renal and bladder ultrasound is the preferred first-line test because it uses sound waves rather than ionising radiation. It can show hydronephrosis, some kidney or ureteric stones, ureteric jets and alternative diagnoses. Transvaginal ultrasound may help identify a distal ureteric stone.

Ultrasound has limitations: physiological hydronephrosis can resemble obstruction, and a small ureteric stone may not be visible. A normal or inconclusive scan does not always exclude a stone.

2. MRI or MR urography when uncertainty remains

MRI without gadolinium can identify the level and pattern of obstruction without ionising radiation. Stones usually appear as signal voids rather than being seen as clearly as on CT. MRI is therefore a useful second-line test, particularly when ultrasound is inconclusive and the patient is clinically stable. Gadolinium contrast is not routinely used during pregnancy.

3. Low-dose non-contrast CT only when clinically necessary

Low-dose CT is the most accurate test for a urinary stone, but it uses ionising radiation. European Association of Urology guidance places low-dose CT as a last-line option in pregnancy, after ultrasound and usually MRI, when diagnostic uncertainty could change urgent management.

The practical radiation principles are:

• use imaging only when it will answer an important clinical question;
• prefer ultrasound, followed by MRI without contrast, when suitable;
• if CT is necessary, use a pregnancy-adapted low-dose protocol and limit the scanned area;
• avoid repeated or multiphase CT examinations unless clearly justified; and
• involve an experienced radiologist and document the risk–benefit discussion.

Diagnostic imaging should not be withheld when delay or uncertainty poses a greater risk to the mother or baby. The EAU notes that deterministic fetal effects require substantially higher exposure than most diagnostic studies and considers doses below 50 mGy safe in this context; nevertheless, any ionising-radiation examination must be justified and kept as low as reasonably achievable. Ultrasound and MRI do not use ionising radiation.

Management options

Treatment is individualised by symptoms, infection status, stone size and location, kidney function, gestational age, obstetric factors and local expertise. Close collaboration between urology, obstetrics, radiology, anaesthesia and neonatology may be needed.

Conservative management

Most uncomplicated cases are initially managed without surgery. This may include:

• oral or intravenous fluids sufficient to correct dehydration, forced overhydration does not “flush out” a stone;
• anti-nausea medication;
• analgesia suitable for the stage of pregnancy;
• urine culture and pregnancy-compatible antibiotics when infection is confirmed; and
• follow-up to ensure symptoms resolve and kidney drainage and function remain satisfactory.

Paracetamol is commonly used first-line. Opioids may be used for severe pain under medical supervision. Non-steroidal anti-inflammatory drugs such as ibuprofen and diclofenac should not be self-administered in pregnancy; their fetal renal, amniotic-fluid and ductus arteriosus risks depend on gestation, dose and duration. The evidence for medical expulsive therapy with alpha-blockers during pregnancy is limited, and such treatment should only be considered after specialist discussion.

Conservative treatment is unsuitable when there is sepsis, deteriorating kidney function, a solitary obstructed kidney, bilateral obstruction, persistent vomiting, uncontrolled pain, severe or progressive hydronephrosis, renal forniceal rupture, threatened premature labour or failure of the stone to pass with ongoing clinically important obstruction.

Ureteric stent

A JJ stent bypasses the obstruction and drains urine from the kidney to the bladder. It can be inserted with minimal or no fluoroscopy, using ultrasound guidance where appropriate. Stents can cause urinary frequency, urgency, discomfort, blood in the urine, infection and encrustation. Encrustation develops faster in pregnancy, so regular review and sometimes exchange every 4–6 weeks are required. A stent must not be forgotten after delivery.

Percutaneous nephrostomy

A nephrostomy tube drains the kidney through the back and can often be placed with ultrasound guidance. It is particularly useful when retrograde stenting is not possible or when urgent drainage is required in selected circumstances. Disadvantages include an external bag, discomfort, blockage, displacement, infection and rapid encrustation, with possible repeat exchanges.

Ureteroscopy and laser treatment

Ureteroscopy allows a surgeon to pass a fine telescope through the bladder into the ureter, remove the stone or fragment it with a laser, often without fluoroscopy. It provides definitive treatment and avoids prolonged drainage in selected patients. If a non-urgent procedure is required, the second trimester is generally preferred, and it should be performed by an experienced team with obstetric and neonatal support available. Ureteroscopy is not the first step in an untreated septic obstruction; drainage and infection control come first.

Treatments generally avoided

Shock-wave lithotripsy is contraindicated during pregnancy. Percutaneous stone removal is rarely required and is reserved for carefully selected cases in highly experienced centres. Routine definitive treatment can often wait until after delivery when symptoms and kidney function permit.

After delivery

Follow-up should confirm that hydronephrosis has resolved, any stent or nephrostomy has been removed, the stone has passed or been treated, and kidney function is normal. A retrieved stone should be analysed. Women with recurrent stones, a strong family history, infection stones or other risk factors may benefit from a metabolic evaluation after pregnancy, when physiology and diet have stabilised.

The important message

Most hydronephrosis in pregnancy is physiological, and many stones pass with careful conservative management. The dangerous combination is obstruction plus infection. Fever, rigors or systemic illness with flank pain requires urgent assessment because prompt antibiotics and drainage can protect the mother, kidney and pregnancy. Imaging should follow a stepwise approach, ultrasound first, MRI second and low-dose CT only when the clinical benefit justifies it.

References

1. European Association of Urology. EAU Guidelines on Urolithiasis: Diagnostic imaging during pregnancy; management of urinary stones during pregnancy. Current online guideline. https://uroweb.org/guidelines/urolithiasis/chapter/guidelines
2. Lee MS, Fenstermaker MA, Naoum EE, et al. Management of nephrolithiasis in pregnancy: multi-disciplinary guidelines from an academic medical center. Front Surg. 2021;8:796876. https://doi.org/10.3389/fsurg.2021.796876
3. Chan K, Shakir T, El-Taji O, et al. Management of urolithiasis in pregnancy. Curr Urol. 2023;17(1):1–6. https://doi.org/10.1097/CU9.0000000000000181
4. American College of Obstetricians and Gynecologists. Guidelines for diagnostic imaging during pregnancy and lactation. Committee Opinion No. 723. Obstet Gynecol. 2017;130–e216. https://www.acog.org/clinical/clinical-guidance/committee-opinion/articles/2017/10/guidelines-for-diagnostic-imaging-during-pregnancy-and-lactation
5. Drescher M, Blackwell RH, Patel PM, et al. Antepartum nephrolithiasis and the risk of preterm delivery. Urolithiasis. 2019;47:441–448. https://doi.org/10.1007/s00240-018-1085-3

This information is general education and does not replace individual medical or obstetric advice. Medication and imaging decisions in pregnancy should be made with the treating obstetric, urology and radiology teams.