Written by:

Dr Matthew Holmes

Dr Holmes is a junior doctor currently based in a NHS trust in North West London. He has completed recent placements in medical microbiology, public health medicine, and A&E (accident and emergency). His interests include the use of technology to improve people's experience and access to healthcare.

Urinary tract infection (UTI) and asymptomatic bacteriuria (ASB) in pregnancy

In this article:

  • The background
  • Definitions:
  • How do urinary tract infections occur?
  • Urinary tract infection in pregnancy
  • Asymptomatic bacteriuria
  • How will I know if I have ASB?
  • How is ASB treated?
  • Can ASB occur again?
  • Conclusion
  • Main points
  • References

Urinary tract infection (UTI) and asymptomatic bacteriuria (ASB) in pregnancy

The background

Urinary tract infections (UTIs – infections of the urethra, bladder, ureters or kidneys) are one of the most common infections affecting women, both during and outside of pregnancy.1 Whilst infections of the urinary tract are no more common in pregnancy, they more readily progress into potentially serious infections.2  This is due to hormonal, anatomical and mechanical changes that occur in pregnancy, which pre-dispose to urinary stasis and reflux (problems with flow of urine).2,3


  • Urinary tract infection – This is the presence of greater than 105 organisms per millilitre of urine, with or without pus cells in the urine
  • Asymptomatic bacteriuria (ASB) – bacterial growth and colonisation in the urinary tract without signs of infection
  • Cystitis – symptomatic infection of the lower urinary tract, usually causing urinary frequency, pain when passing urine, an urgent need to pass urine, night-time urination and strong smelling urine
  • Pyelonephritis – infection of the upper urinary tract (kidneys and ureters). Leads to pain in the loin/flank, fever, nausea and vomiting

How do urinary tract infections occur?

The urinary tract, which is normally sterile, may become infected as the result of ascending colonisation of bacteria naturally occurring in the skin, vaginal or colonic flora.  As the bacteria ascend, symptoms affecting the lower urinary tract, commonly called cystitis, may develop. The natural defence mechanisms of the urinary tract may contain the infection here; if this does not happen, the infection may spread to the upper part of the urinary tract, causing pyelonephritis.

However, infection of the urinary tract can present with or without symptoms; if no symptoms of an infection are experienced then this is referred to as asymptomatic bacteriuria (ASB).

Women are more prone to infections of the urinary tract then men because of a shorter urethra and closer proximity of the urethra to potentially disease-causing bacteria.

Urinary tract infection in pregnancy

A number of changes occur in the urinary tract in pregnancy. Higher levels of the hormone progesterone cause dilatation of the upper parts of the urinary tract around the kidneys and the connecting tubes between the kidneys and bladder (the ureters). This can promote flow of urine in the wrong direction, referred to as urinary reflux. The growing uterus also reduces drainage of the bladder, making it more difficult to empty the bladder (known as urinary stasis).3  Increasing levels of glucose and amino acids in the urine, and changes in urinary pH, may also provide bacteria with a more favourable environment for growth and spread.4  All these factors increase the likelihood of an infection ascending to the higher part of the urinary tract and causing pyelonephritis.

The biggest risk factor for development of UTI in pregnancy is previous infections of the urinary tract outside of pregnancy. Additional attention should be given for the care of women who have experienced multiple infections in the past.

It is important that if you experience any symptoms of cystitis or pyelonephritis that you discuss and investigate these symptoms with a doctor. Your doctor will take a urine sample and perform a urine dipstick to look for signs of infection. They may also send the urine sample for further analysis at a laboratory, where operatives will undertake urine microscopy, culture and sensitivity to identify the specific bacteria causing infection. An infection should then be treated with appropriate antibiotics.

Simple cystitis will require 3-7 days of antibiotic treatment. Oral antibiotics are usually appropriate. Pyelonephritis normally requires admission to hospital for intravenous antibiotics and careful monitoring. A longer course of antibiotics (10-14 days) is usually required.

Asymptomatic bacteriuria

In pregnancy between 2-10% of women will experience infection of the urinary tract without any symptoms, known as asymptomatic bacteriuria (ASB).4  ASB occurs commonly in the general population, increasing in frequency as people increase in age.5  Studies in the general population show that there is no benefit in treating ASB; however, in pregnancy ASB is important because it increases the risk of pre-term (early) labour and maternal pyelonephritis, conveying significant morbidity (risk of further illness or complication) to both mother and baby.6-9  Studies have indicated that if untreated ASB will lead to pyelonephritis in 30% of cases.10  It is therefore vital to identify and treat affected mothers.

How will I know if I have ASB?

As there are no symptoms, ASB screening should be undertaken as part of routine antenatal care. The National Institute for Clinical Excellence (NICE) recommends screening for ASB in early pregnancy.11 This will usually be performed at your initial booking appointment, as many cases of urinary tract infection occur in the first trimester.3

Importantly, NICE recommends urine culture (sending a sample to a lab) rather than simply performing urine dipstick testing. This is because a urine dipstick will not detect all cases of ASB (urine dipstick has a lower sensitivity). Urine culture is more likely to detect the presence of pathogenic bacteria.12,13

One positive urine culture test does not necessarily indicate that you have ASB; it may be appropriate to repeat a urine culture after one positive result to confirm the diagnosis. This is because 80% of women will provide a single positive culture result. If a repeat sample is positive, the chance of ASB increases to 95%.8

How is ASB treated?

ASB can be treated with a simple course of antibiotics. A recent Cochrane systematic review has found that a four-to-seven day duration of treatment is recommended, although further studies are required to determine the effectiveness of shorter (three- to five day) antibiotics against a full week course.14

Antibiotics that are safe in pregnancy and appropriate for treatment of urinary tract infections include:

  • Cefalexin
  • Amoxicillin
  • Co-amoxiclav
  • Nitrofurantoin – this antibiotic is safe throughout the majority of pregnancy, but is to be avoided at full term, as it can lead to neonatal haemolysis (breakdown of red blood cells in the newborn baby).

Other antibiotics may be considered in the case of allergies or intolerances to the above medications.

Can ASB occur again?

Although ASB can be effectively treated, it is possible that it will return; indeed those who have had ASB are more prone to developing it again. It is therefore recommended that a repeat urine culture sample be sent one to two weeks after treatment, to ensure complete eradication of bacteria. Following this, women who have been diagnosed and treated for ASB should have monthly urine culture samples to look for relapse.3


Although UTI is no more common in pregnancy, it can lead to more serious infections (pyelonephritis) and can lead to significant morbidity for both mother and baby. UTI can present both with and without symptoms and it is therefore important to screen for ASB; this is normally undertaken in the first trimester (when UTI is most common).

ASB, like other UTIs, can be treated with a course of antibiotics. Regular follow-up is advised in women who have been diagnosed with ASB.

Main points

  • Due to hormonal, anatomical and mechanical changes occurring in pregnancy urinary tract infections can more easily turn into potentially serious infections.
  • A UTI may show no symptoms, but if left untreated can cause early labour and will increase the risk of complications and further illness.
  • Any symptoms of cystitis and infection of the upper urinary tract (pyelonephritis) such as pain, fever, nausea and vomiting should be discussed with your GP or midwife as soon as possible.
  • In pregnancy between 2-10% of women will experience infection of the urinary tract without any symptoms – asymptomatic bacteriuria (ASB).
  • NICE recommends routine screening for ASB in pregnant women during early pregnancy.
  • ASB screening is often carried out by a midwife at the booking-in appointment.
  • ASB can be treated with a course of antibiotics – there are several medications suitable for pregnant women.
  • It is possible for ASB to return after treatment, so it is recommended that follow up screening occurs one to two weeks after treatment and regular follow up tests in case of relapse.


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