Kidney Infection Symptoms in Pregnancy: A Complete Guide

Feeling feverish, shivery, and sore on one side of your back during pregnancy is not something to put down to normal aches. These are classic kidney infection symptoms in pregnancy and they need medical attention the same day.

A kidney infection, known medically as pyelonephritis, is one of the most common reasons pregnant women are admitted to hospital outside of obstetric emergencies. It affects around 1–2% of pregnant women and carries real risks for both mother and baby. Getting to know the signs, understanding why pregnancy raises the risk, and knowing when to act can make all the difference.

Here is everything you need to know.

Why Pregnancy Increases the Risk of Kidney Infection

Your body changes in ways that make the urinary tract more vulnerable during pregnancy. Let's break it down.

Pregnancy causes anatomical and physiological changes that increase susceptibility to urinary tract infections (UTIs). Progesterone relaxes smooth muscle, which slows the flow of urine, and the growing uterus compresses the bladder, reducing its capacity and leading to incomplete emptying. Urine that sits in the bladder for longer gives bacteria a chance to multiply.

The immune system also works differently during pregnancy. It adjusts to tolerate the baby, which means it is less aggressive at fighting off bacterial infections elsewhere in the body.

Urine during pregnancy contains more protein, hormones, and glucose than usual, which creates conditions that encourage bacterial growth.

Most kidney infections during pregnancy start as a lower urinary tract infection, a bladder infection or cystitis  that spreads upward to the kidneys. If asymptomatic bacteriuria goes untreated, 30% of women may develop acute pyelonephritis. This is why your midwife checks your urine at every antenatal appointment, even if you feel completely well.

The period between six and 24 weeks of pregnancy carries the highest risk for urinary infections because of the speed at which the uterus is growing and the hormonal shifts taking place.

Kidney Infection Symptoms in Pregnancy: What to Look For

Kidney infection symptoms in pregnancy can appear suddenly sometimes within a few hours. Do not wait for every symptom to be present before calling your GP or midwife. One or two warning signs are enough to act.

The main symptoms of a kidney infection during pregnancy include:

• Pain or aching in one or both sides of your back, just below your ribs (flank or loin pain)

• A high temperature, usually above 38°C

• Shivering or shaking uncontrollably (rigors)

• Feeling or being sick (nausea and vomiting)

• Pain or burning when you pass urine

• Needing to wee more often than usual, or with greater urgency

• Cloudy, dark, or strong-smelling urine

• Blood in the urine

• General feeling of being very unwell, even if you cannot pinpoint exactly why

Symptoms usually develop quickly over a few hours and the infection will almost always cause some symptoms of a bladder infection alongside the flank pain and fever.

It is worth knowing that symptoms may not immediately suggest a UTI at all, which is part of why kidney infections in pregnancy sometimes go unrecognised until they become serious. If you feel suddenly and severely unwell at any point in your pregnancy, contact your midwife or GP without delay.

How Is a Kidney Infection Different from a Bladder Infection?

Here is a quick way to tell them apart.

A bladder infection (cystitis) usually causes burning when you wee, needing to go more often, and perhaps some lower abdominal discomfort. You may feel unwell, but it tends to be manageable.

A kidney infection goes further. The back or flank pain, the high fever, the shaking and vomiting these signal that infection has moved beyond the bladder. Untreated pyelonephritis can lead to severe maternal complications including septic shock and acute respiratory distress syndrome (ARDS).

In pregnancy, the gap between a bladder infection and a kidney infection can close faster than it would at any other time in your life. This is why treating any urinary symptoms promptly matters so much.

When to Seek Emergency Help

Call 999 or go to A&E immediately if you have any of the following:

• Difficulty speaking or feeling confused or very drowsy

• Rapid breathing or feeling short of breath

• Severe shaking or uncontrollable shivering

• Skin that looks pale, mottled, or blotchy

• No urine output or very reduced urine output

These are signs of sepsis. Women with pyelonephritis in pregnancy face a significantly higher risk of septicaemia compared to those without the infection. Sepsis moves fast and needs emergency treatment.

Call your GP, midwife, or NHS 111 the same day if you have:

• Fever with back or flank pain during pregnancy

• High temperature (above 37.5°C) with shivering

• Nausea or vomiting alongside urinary symptoms

• Pain in your lower back that is worsening

• Blood in your urine

The NHS advises calling 111 if you cannot get to see a GP and your symptoms are getting worse. During pregnancy, do not hold off to see whether symptoms settle on their own.

What Happens When You See a Doctor

Your doctor or midwife will act quickly. Here is the process.

Urine testing comes first. You will be asked to provide a midstream urine (MSU) sample. A dipstick test gives an immediate result, but the sample is also sent to a laboratory so the bacteria can be identified. This helps doctors prescribe the right antibiotic.

Blood tests may be ordered to check whether the infection has reached the bloodstream and to assess kidney function.

Antibiotics are started straight away, often before laboratory results are back. The main treatment for a kidney infection is an antibiotic course, typically lasting 7 to 14 days. For pregnant women, the choice of antibiotic takes your stage of pregnancy and fetal safety into account. Some antibiotics used in kidney infections are given intravenously (by drip) in hospital if you are very unwell.

Severe pyelonephritis during pregnancy is treated in hospital with intravenous antibiotics. Most women improve within 48 to 72 hours of starting treatment, though the full course must be completed.

Monitoring during a hospital stay includes checking urine output, temperature, and blood pressure. Your baby's heart rate may also be monitored depending on your stage of pregnancy.

After treatment, your midwife or doctor will test your urine again to confirm the infection has cleared. If you have recurring infections during pregnancy, a preventative (prophylactic) low-dose antibiotic course may be considered.

Risks to Mother and Baby If Left Untreated

This is the part that makes kidney infection symptoms in pregnancy so important to act on. Left untreated, the consequences can be serious.

For the mother:

Pulmonary complications occur in up to 10% of pregnant patients receiving treatment for pyelonephritis, due to endotoxin-mediated alveolar damage that can present as pulmonary oedema or acute respiratory distress syndrome.

Around 25% of pregnant women with pyelonephritis have a decreased glomerular filtration rate, indicating temporary kidney dysfunction. In most cases this resolves with treatment, but it shows how quickly the kidneys can be affected.

For the baby:

Antenatal pyelonephritis is associated with preterm premature rupture of membranes, preterm labour, and low birth weight.

Preterm delivery occurs in around 11% of cases, and birth weight below 2,500g is seen in approximately 8% of cases.

Getting treatment early dramatically reduces these risks. The team at myGynaePlus regularly reminds patients that prompt reporting of any urinary symptoms, however mild, is one of the most protective things you can do during pregnancy. Do not wait for symptoms to become severe.

How to Reduce Your Risk of Kidney Infection During Pregnancy

You cannot always prevent a kidney infection, but these steps lower your chances considerably.

Stay hydrated. Drinking at least 6–8 glasses of water per day keeps urine pale yellow, which shows you are well hydrated. Concentrated urine is a better environment for bacteria to thrive.

Do not hold your urine. Go to the toilet as soon as you feel the urge. Holding in urine allows bacteria to stay in the bladder longer.

Wipe front to back. This is one of the most effective ways to stop gut bacteria from reaching the urethra.

Pass urine after sex. This flushes bacteria out of the urethra before they can travel upward.

Attend all your antenatal appointments. Your midwife screens your urine for bacteria at every visit even when you feel completely well. Routine urine screening during pregnancy catches infections before they cause symptoms, giving doctors the chance to treat them early.

Avoid scented intimate products. Scented soaps, bubble baths, and vaginal washes can alter the pH of the area around the urethra and make infection more likely. Plain water is enough.

Treat constipation promptly. Constipation can increase the chances of a bladder or kidney infection because of the pressure the bowel places on surrounding structures.

Kidney Infection and Specific Trimesters: What Changes

The risk is not equal across all three trimesters.

First trimester: Hormonal changes begin to relax the muscles of the urinary tract, and the uterus starts to grow. Asymptomatic bacteriuria is most likely to be picked up during early booking appointments.

Second trimester: The median gestational age at which antenatal pyelonephritis presents is around 27 to 28 weeks, with 51% of cases presenting before 28 weeks' gestation. The uterus is large enough at this point to put real pressure on the bladder and ureters.

Third trimester: The baby's weight adds more pressure to the bladder, making it harder to empty completely. Group B Streptococcus (GBS) becomes a more common urinary pathogen as the pregnancy progresses, alongside E. coli.

Getting Specialist Care for Recurring Urinary Symptoms in Pregnancy

If you have had repeated UTIs or you are worried about symptoms during your pregnancy, speaking to a specialist is a sensible step. The consultants at myGynaePlus offer antenatal care that includes thorough assessment of urinary symptoms as part of personalised pregnancy management. Specialist input can be particularly useful if you have had a previous kidney infection in pregnancy, gestational diabetes, or a history of recurrent UTIs.

Early assessment means early treatment and that protects both you and your baby.

FAQs: Kidney Infection Symptoms in Pregnancy

1. Can a kidney infection in pregnancy harm my baby? 

Yes, if left untreated. Kidney infections in pregnancy are linked to preterm birth, low birth weight, and premature rupture of membranes. The risks are reduced substantially when treatment starts early. If you notice symptoms, contact your GP or midwife the same day rather than waiting to see whether things improve.

2. How quickly do kidney infection symptoms appear during pregnancy? 

Symptoms can develop within a few hours. You may feel fine in the morning and have a high fever, back pain, and vomiting by the afternoon. This rapid onset is one of the reasons kidney infection in pregnancy needs to be treated urgently and do not delay seeking help if symptoms appear.

3. Is back pain during pregnancy always a sign of a kidney infection? 

Not always. Back pain is very common in pregnancy for postural and musculoskeletal reasons. Kidney infection pain tends to be felt in the flank or loin area  the side of your back below the ribs rather than low in the lumbar region. It is usually accompanied by fever, nausea, or urinary symptoms. If you are unsure, contact your midwife.

4. Are antibiotics safe for treating a kidney infection during pregnancy? 

Yes. Doctors prescribe antibiotics that are safe for both mother and baby throughout pregnancy. The choice of antibiotic depends on the bacteria identified in your urine sample and your stage of pregnancy. Some first-line antibiotics are adjusted in the third trimester. Always complete the full course, even if you feel better quickly.

5. What happens if a kidney infection during pregnancy does not respond to antibiotics? 

If your symptoms do not improve within 24 hours of starting antibiotics, your doctor may consider hospital admission for intravenous treatment. Your urine culture results will guide a switch to a more targeted antibiotic if the bacteria show resistance to the initial prescription. Further investigation such as an ultrasound of the kidneys may also be arranged.