What Happens If You Have a Kidney Infection During Pregnancy?

A kidney infection during pregnancy is not something to monitor from home and hope it passes. It is one of the most common reasons pregnant women are admitted to hospital in the UK, and without prompt treatment, it can lead to serious complications for both mother and baby.

The good news is that with early diagnosis and the right treatment, most women recover fully. The challenge is knowing what to look for, acting quickly, and understanding why pregnancy makes this particular infection more serious than it would be at any other time.

Here is everything you need to know.

What Is a Kidney Infection in Pregnancy?

A kidney infection, known medically as pyelonephritis, occurs when bacteria travel from the bladder up into the kidneys. It is a type of urinary tract infection (UTI), but one that has progressed beyond the bladder to affect the upper urinary tract.

Most kidney infections during pregnancy are caused by Escherichia coli (E. coli), the same bacteria responsible for the majority of bladder infections. In some cases, other organisms such as Klebsiella pneumoniae or Group B streptococcus are involved.

Kidney infections affect roughly 1 to 2% of pregnant women. That may sound small, but given how many pregnancies there are, it is one of the most frequent serious bacterial infections doctors see in maternity care. According to the American College of Obstetricians and Gynecologists (ACOG), pyelonephritis is one of the leading causes of hospitalisation during pregnancy.

Why Pregnancy Increases the Risk

Your body goes through major physical changes during pregnancy that make the urinary tract more vulnerable to infection. Let's break it down.

Progesterone relaxes smooth muscle. This includes the muscles lining the ureters (the tubes connecting your kidneys to your bladder). When those muscles relax, urine moves more slowly. Bacteria thrive in slow-moving urine.

Your uterus compresses the ureters. As the uterus grows, it puts direct pressure on the ureters, partly blocking urine flow. This creates pockets where bacteria can sit and multiply.

Urine composition changes. During pregnancy, urine becomes less acidic and contains higher levels of glucose, protein, and hormones. These changes make it a better breeding ground for bacteria.

The immune system is partially suppressed. Your body naturally reduces certain immune responses to protect the baby from being rejected. This means infections can take hold more easily and spread further before your body mounts a defence.

The kidneys themselves enlarge. Pregnancy causes both kidneys to expand and the renal collecting system to dilate, a condition called hydronephrosis of pregnancy. This dilation allows bacteria more room to travel upwards.

The result is that bacteria which might cause a straightforward bladder infection in a non-pregnant person can progress to the kidneys far more quickly during pregnancy.

How a Kidney Infection Develops: The Progression

Not every kidney infection arrives out of nowhere. There is usually a chain of events. Here is how it typically unfolds.

Step one: Asymptomatic bacteriuria. This is when bacteria are present in the urine but cause no symptoms. It affects around 2 to 10% of pregnant women. You feel completely normal. Without screening, you would have no idea.

This matters because, left untreated, asymptomatic bacteriuria progresses to pyelonephritis in a substantial number of cases. This is why routine urine screening at the first antenatal appointment exists. Finding and treating silent bacteria early prevents many kidney infections from ever happening.

Step two: Bladder infection (cystitis). If asymptomatic bacteriuria is missed or left untreated, bacteria can multiply and cause a symptomatic bladder infection. Symptoms include burning when urinating, urgency, frequency, and lower abdominal discomfort.

Step three: Kidney infection (pyelonephritis). Bacteria from the bladder travel up the ureters to one or both kidneys. At this point, the infection becomes systemic, meaning it affects your whole body, not just your urinary tract.

Symptoms of a Kidney Infection During Pregnancy

Kidney infection symptoms can appear quickly, sometimes within hours of feeling unwell. They are more intense than a typical bladder infection and usually include symptoms that extend beyond the urinary tract.

Symptoms to look out for:

• Fever, often above 38°C, sometimes accompanied by chills and shaking (rigors)

• Pain in the flank (the side of your back, below the ribs on one or both sides)

• Lower back pain that is constant rather than coming and going

• Pain or burning when urinating

• Needing to urinate frequently, with little coming out

• Cloudy, dark, or strong-smelling urine

• Nausea and vomiting

• Abdominal pain

• Feeling very unwell, weak, or confused

According to a study published in BMC Pregnancy and Childbirth via PubMed Central, the most common clinical signs in pregnant women with pyelonephritis include fever, loin pain, and abdominal pain, with right-sided pain occurring in around 67% of cases.

One important point: you may not have classic urinary symptoms at all. Fever and flank pain on their own, without any burning or urgency when urinating, can still signal a kidney infection during pregnancy.

When to call for help immediately:

Call your GP, midwife, or maternity unit right away if you develop a fever alongside back or flank pain during pregnancy. Do not wait to see if it settles. If you develop a very high fever, severe pain, confusion, or feel genuinely unwell, go to A&E or call 999.

What Happens If a Kidney Infection Goes Untreated?

This is where urgency matters most. An untreated kidney infection in pregnancy carries real risks to both mother and baby.

Preterm labour. The infection triggers uterine contractions in some women. A study in Obstetrics and Gynecology (2005) found that 32% of pregnant women hospitalized with pyelonephritis experienced threatened preterm labour, and 17% delivered preterm. Bacteria and the body's inflammatory response to infection can irritate the uterus and set off labour early.

Low birth weight. Babies born to mothers with untreated kidney infections are at increased risk of being born underweight (below 2,500g), which brings its own complications after birth.

Sepsis. If bacteria enter the bloodstream from the kidneys, sepsis can develop. This is a life-threatening emergency. Sepsis occurs in up to 17% of pyelonephritis cases in pregnancy, according to clinical guidance from FP Notebook. Signs include a very high or very low temperature, rapid heart rate, rapid breathing, and confusion.

Acute respiratory distress syndrome (ARDS). In severe cases, fluid can accumulate in the lungs, making it difficult to breathe. This is a rare but serious complication that requires intensive care.

Anaemia. The toxins produced by some bacteria (endotoxins) can break down red blood cells, leading to anaemia that may need treatment during or after the pregnancy.

Recurrent infection. Up to 23% of pregnant women who develop pyelonephritis once will have a recurrence during the same pregnancy, according to the American Academy of Family Physicians (AAFP). This is why treatment is followed by monitoring and, in some cases, low-dose preventive antibiotics for the remainder of the pregnancy.

How a Kidney Infection Is Diagnosed

Diagnosis involves a urine test and sometimes blood tests.

Urinalysis checks your urine for white blood cells, red blood cells, and bacteria. Results come back quickly and help your doctor make an initial assessment.

Urine culture identifies the specific bacteria causing the infection and tests which antibiotics it responds to. This takes 24 to 48 hours but guides the choice of antibiotic if initial treatment is not working.

Blood tests check for signs of systemic infection (raised white blood cell count, elevated inflammatory markers) and assess kidney function.

Renal ultrasound may be carried out if symptoms do not improve within 24 to 48 hours of starting treatment, or if there is any concern about a structural problem with the kidneys.

Treatment for Kidney Infection During Pregnancy

The approach to treatment depends on how unwell you are.

Hospital admission. Most pregnant women with pyelonephritis are admitted to hospital, at least initially. The ACOG clinical consensus recommends inpatient management given the risks associated with this condition during pregnancy.

Intravenous antibiotics. Treatment starts with antibiotics given directly into a vein. This is faster and more reliable than tablets when the infection is severe. Commonly used antibiotics include cephalosporins such as cefazolin or ceftriaxone. Fluoroquinolones are generally avoided during pregnancy due to potential effects on fetal development.

Switching to oral antibiotics. Once you are improving (usually after 48 to 72 hours and when your temperature has been normal for 24 hours), treatment switches to antibiotic tablets. The full course typically runs for 10 to 14 days in total.

Fluids. Intravenous fluid support helps maintain kidney function and keeps you hydrated if you have been vomiting.

Fever management. Paracetamol is safe in pregnancy and is used to manage high temperatures. Ibuprofen and other anti-inflammatory drugs should be avoided unless specifically recommended by a doctor.

Ongoing monitoring. Your baby's heart rate and your uterine activity will be monitored during your stay. If contractions develop, the medical team will assess whether treatment for preterm labour is needed.

Suppressive antibiotics. After completing treatment, some women are prescribed a low daily dose of antibiotics for the remainder of the pregnancy to prevent the infection returning. This decision depends on individual circumstances and risk factors.

Preventing Kidney Infections During Pregnancy

Prevention starts at the booking appointment.

Urine screening at the first antenatal visit. This is standard practice in UK maternity care and is one of the most effective ways to prevent kidney infections. Detecting and treating silent bacteria (asymptomatic bacteriuria) early stops it progressing. Research consistently shows that treating asymptomatic bacteriuria in pregnancy reduces the incidence of pyelonephritis significantly.

Treating bladder infections promptly. If you develop any urinary symptoms during pregnancy, get tested and treated without delay. A bladder infection that clears up quickly with antibiotics cannot progress to the kidneys.

Daily habits that reduce infection risk:

• Drink enough water throughout the day (around 6 to 8 glasses)

• Urinate when you feel the urge rather than holding on

• Wipe front to back after using the toilet

• Urinate before and after sex

• Avoid scented soaps, bubble baths, and wipes around the genital area

• Wear breathable, cotton underwear

If you have had a kidney or bladder infection before, tell your midwife or gynaecologist at your first appointment. Previous UTIs, diabetes, and structural abnormalities of the urinary tract all increase the risk and may mean more frequent urine monitoring throughout your pregnancy.

Getting Support and Assessment

At myGynaePlus, the specialist team in West London provides antenatal care, obstetric consultations, and women's health assessments throughout pregnancy. If you are experiencing urinary symptoms, back pain, or fever at any stage of pregnancy, getting checked quickly makes a real difference to outcomes for you and your baby.

Early assessment means earlier treatment. That is the most effective protection against the complications a kidney infection can bring.

If you are not sure whether your symptoms need attention, contact your GP, midwife, or the team at myGynaePlus rather than waiting to see how things develop.

Frequently Asked Questions

Can a kidney infection cause a miscarriage?

A kidney infection is more strongly associated with preterm labour than miscarriage, but any serious infection during pregnancy carries risk if untreated. The connection between pyelonephritis and adverse pregnancy outcomes, including preterm birth and low birth weight, is well established in obstetric research. Prompt treatment significantly reduces these risks. If you are in early pregnancy and develop fever and back pain, seek assessment the same day.

How do I know if my back pain is from a kidney infection or normal pregnancy discomfort?

Normal pregnancy-related back pain tends to be a dull, muscular ache that comes and goes, often worse after standing or sitting for a long time. Kidney infection pain typically sits below the ribs on one or both sides (the flanks), feels constant, and is accompanied by fever, nausea, or urinary symptoms. If you have back pain with any fever at all during pregnancy, contact your midwife or GP promptly.

Is it safe to take antibiotics for a kidney infection during pregnancy?

Yes. Certain antibiotics are safe to use during pregnancy and are the standard treatment for kidney infections. Your doctor will prescribe a pregnancy-safe option such as cefazolin or cefalexin. Completing the full course is important, even after you start feeling better. Never stop antibiotics early during pregnancy without speaking to your doctor first.

Will a kidney infection affect my baby long-term?

With timely treatment, most babies are unaffected. The main risks from an untreated kidney infection are preterm delivery and low birth weight. Both of these risks reduce substantially when the infection is caught early and treated with antibiotics. Your baby will likely be monitored during and after your hospital stay to ensure everything is progressing normally.

How soon will I feel better after starting treatment?

Most women begin to feel better within 48 to 72 hours of starting intravenous antibiotics. The fever usually drops within the first 24 hours of treatment. You will need to complete the full 10 to 14 day antibiotic course even after symptoms resolve. If you are not improving within 48 hours, your doctor will repeat tests and may consider further investigations such as a kidney scan.