Group B Strep Myth Buster

Group B Streptococcus (also known as ‘group B Strep’ or ‘GBS’) is a natural bacterium that is commonly found in the intestines of up to one in every three adults as well as in the vaginas of one in four women. Group B Strep carriage is not an infection; it is some of the natural bacteria that live on and in us. It is defined as a ‘commensal’, an organism that lives in or on another without causing harm. Carriage is asymptomatic, without signs or symptoms.


Why you should be group B Strep aware…

Even though group B Strep is a natural and normal bacterium, it is the most common cause of life-threatening infection in newborn babies, and of meningitis in babies up to three months. At least 500 babies a year in the UK become infected, most commonly with early-onset group B Strep infection (developing in the first 6 days of life) and less frequently late-onset group B Strep infection (developing in babies aged 7-90 days). Newborn babies do not have well developed immune systems and are more susceptible to bacteria. Overall, the risk of a baby developing a group B Strep infection without preventative medicine is:

  • 1 in 1,0001 where the woman’s group B Strep carriage status is not known;
  • 1 in 400 where the woman is carrying group B Strep during the pregnancy;
  • 1 in 300 where the woman is carrying group B Strep at delivery; and
  • 1 in 100 where the woman has had a previous baby infected with group B Strep.

The following are the most common myths about group B Strep; including misconceptions about testing and prevention.

Myth Busting

Myth 1 – We test all pregnant women’s urine, so if she’s a carrier group B Strep, it will be detected then.

No. Urine testing is not an accurate indicator of group B Strep carriage. It can only detect group B Strep from a urine sample, not when it is present in the vagina and/or rectum.

As urine is normally sterile inside the bladder, when group B Strep is detected in a urine sample, this is often indicating a urinary tract infection is present. Other times, particularly when the level detected is low, it indicates carriage rather than infection – the urine has simply picked up some group B Strep bacteria from vaginal carriage on its way out of the body. However, much more frequently, despite maternal vaginal and/or rectal carriage, group B Strep is not detected in the urine sample at all so carriage is missed completely.

When a urine sample from a pregnant woman grows group B Strep ≥10^5 cfu/ml, whether mum has symptoms of a urine infection or not, it is important to treat the pregnant woman with antibiotics. If left untreated, such infections can cause kidney damage and have been linked to preterm labour.

Group B Strep detected from a urine sample (or from a vaginal or rectal swab) at any level during pregnancy means Mum should be offered intravenous antibiotics once labour has started, in addition to any antibiotics given to treat a urinary tract infection.

Myth 3 – There is no point in testing since [group B Strep carriage is transient] OR [‘once a carrier always a carrier.’]

The definition of the word transient is ‘lasting only for a short period of time; usually not recurring.’ It is misleading when people use this word interchangeably ‘comes and goes’ – as in here today gone tomorrow – relating to group B Strep carriage.

Group B Strep carriage can come and go from the vagina quite naturally, but this happens over a period of time, not daily. Thus the 35-37 weeks of pregnancy window is considered optimal for testing. Tests taken then are very good at predicting whether or not Mum will be carrying when she is most likely to go into labour – over the following 5 weeks.4

Group B Strep carriage detected before the current pregnancy is not a good predictor of whether the Mum is carrying the bacteria in a subsequent pregnancy, although research has shown that 38-53% of women carrying group B Strep in one pregnancy are also positive in the next.5-7

Myth 4 – Only mums with a previous baby affected by group B Strep should be offered IV antibiotics in labour.


The Royal College of Obstetricians & Gynaecologists (RCOG) recommends a ‘risk factor’ approach (instead of antenatal screening) to prevent early-onset group B Strep infection (0-6 days of life).

The RCOG guidelines8 recommend that intrapartum antibiotic prophylaxis (IAP) should be offered to Mums as soon as possible from the start of labour in the following instances:

  1. If group B Strep carriage was detected during the current pregnancy from a vaginal and/or rectal swab.
  2. If group B Strep was found during the current pregnancy from a urine sample. (Women with group B Strep urinary tract infection during pregnancy should receive appropriate treatment at the time of diagnosis as well as the offer of intravenous antibiotics in labour.)
  3. If a previous baby developed group B Strep
  4. If Mum has a fever during labour (temperature of greater than 38oC) and/or chorioamnionitis is suspected (in these situations, Mum should be offered broad-spectrum antibiotics that include group B Strep cover).

The RCOG recognises these next two risk factors as increasing a baby’s risk of developing early-onset group B Strep infection but does not specifically recommend the offer of IAP:

  • Labour starting or waters breaking before 37 completed weeks of pregnancy
  • Where there is prolonged rupture of the membranes – more than 18 hours before delivery

The National Institute for Health and Clinical Excellence (NICE), in addition to the first four instances listed above, also includes the following preterm labour scenarios where the use of IAP should be considered9:

  1. For women in preterm labour if there is prelabour rupture of membranes of any duration.
  2. For women in preterm labour if there is suspected or confirmed intrapartum rupture of membranes lasting more than 18 hours.

*The guidelines from the RCOG are due to be reviewed in 2015/2016.

Myth 5 -“Given that most women carry group B Strep without incident, testing and potentially treating everyone with IV antibiotics would cause more deaths from anaphylactic shock–nevermind future antibiotic resistance–than would be saved from group B Strep infection.”

Most women who are group B Strep carriers go on to have healthy babies with no incident. The risk of a baby born to a Mum carrying group B Strep at delivery of developing an early-onset group B Strep infection is around one in 300 when no preventative measures are taken. Testing every pregnant woman does not automatically mean every pregnant woman would be offered (or would want) intravenous antibiotics in labour. Only the women who test positive for group B Strep carriage would be offered that option.

Good data is hard to find on this subject. The following are often quoted though widely-accepted as being over-estimates of the risks of penicillin:

  • 1 in 10 of the mother developing a mild allergic reaction, such as a rash;
  • 1 in 10,000 of the mother developing a severe allergic reaction (anaphylaxis); and
  • 1 in 100,00010 of the mother developing fatal anaphylaxis, resulting in her death.

Research has shown that narrow spectrum intravenous antibiotics (ideally penicillin), given as soon as possible once labour starts to women whose babies are at higher risk of developing group B Strep infection, is highly effective at reducing the risk of group B Strep infection in newborn babies. Such intrapartum antibiotics have not been shown to have long-term side-effects on the baby, nor an apparent tendency to increase antibiotic resistance. Indeed, group B Strep has remained sensitive to penicillin for over 60 years.

If a mum in your care is interested in being tested for group B Strep carriage using the ECM test, Group B Strep Support keeps an up to date page of trusts and private labs which offer the test following PHE’s guidelines.

For more information on all things group B Strep including the most commonly asked questions from health professionals and mums alike, their website is very comprehensive. GBSS also provides free information leaflets, posters, RCOG, NICE, and PHE guidelines, as well as “GBS alert” stickers to health professionals.

1(Reference: Estimated early-onset group B Streptococcal neonatal disease. Luck S, Torny M, d’Agapeyeff K, Pitt A, Heath P, Breathnach A, Russell AB. Lancet, June 2003).



4The accuracy of late antenatal screening cultures in predicting genital group B streptococcal colonization at delivery. Yancey MK, Schuchat A, Brown LK, Ventura VL, Markenson GR. Obstet Gynecol. 1996 Nov;88(5):811-5. Am J Obstet Gynecol. 1984 Apr 1;148(7):915-28.

5 Cheng PJ, Chueh HY, Liu CM, Hsu JJ, Hsieh TT, Soong YK. Risk factors for recurrence of group B streptococcus colonization in a subsequent pregnancy. Obstet Gynecol. 2008 Mar;111(3):704-9. Abstract at

6Page-Ramsey SM, Johnstone SK, Kim D, Ramsey PS. Am J Perinatol. Prevalence of Group B Streptococcus Colonization in Subsequent Pregnancies of Group B Streptococcus-Colonized versus Noncolonized Women.2012 Sep 21. Abstract

7Turrentine MA, Ramirez MM. Recurrence of group B streptococci colonization in subsequent pregnancy. Obstet Gynecol. 2008 Aug;112(2 Pt 1):259-64. Abstract at

8RCOG: Clinical Green Top Guideline No 36 Prevention of early onset neonatal GBS disease (issued 2003, updated 2012)

9 NICE: Antibiotics for the prevention and treatment of early-onset neonatal infection (2012)

10The prevention of neonatal group B streptococcal disease. MR Law, G Palomaki, Z Alfirevic, R Gilbert, P Heath, C McCartney, T Reid, S Schrag on behalf of the Medical Screening Society Working Group on GBS Disease. J Med Screen 2005;12:60-68.

With thanks
In partnership with Group B Strep Support. To find out more visit their website which contains a wealth of information for patients, carers and professionals.