GBS screening: the evidence

Group B Strep (GBS) is a bacteria that many of us carry. Most of the time it doesn’t cause any problems. Very occasionally, a newborn baby can become infected with this bacteria and become ill. Even more occasionally, GBS disease can lead to serious illness or be fatal.

Pregnant women are offered screening and treatment for GBS, but this is controversial. On this page, I’ll explain why, and link to some recent evidence on this topic.

I have written a book called Group B Strep Explained, for those who want to know more about this area, the evidence, and their options.

There are several elements to understanding why GBS screening and treatment is controversial.

As I explained above, GBS disease affects only a very small number of babies, but it can sometimes be serious or fatal.

We can screen women/babies to decide who might be at higher risk, but the rarity of GBS disease and the bluntness of the current screening methods mean that we end up offering unnecessary and potentially harmful treatment (in this case intravenous antibiotics during labour) to hundreds of thousands of women/babies.

We don’t have good evidence that antibiotics are beneficial.

Antibiotics are known to have a number of downsides, including that they negatively affect our microbiomes.

Current approaches to GBS mean that we are giving antibiotics to many thousands of people who don’t benefit from them, when we don’t have evidence that this helps reduce mortality from GBS disease.

Like everything, it’s your decision whether to have screening and/or treatment for GBS, but parents have to weigh up difficult decisions in the absence of good evidence.

This page summarises some of the key studies on this topic in recent years.

If you would like more information, and to explore the wider issues so you can make the decisions that are right for you, see my book, Group B Strep Explained.

GBS and the risk of perinatal morbidity and mortality following term labor

In a paper titled, Group B Streptococcus and the risk of perinatal morbidity and mortality following term labor, Stephens et al (2023) summarise their review of the evidence relating to screening and treatment for GBS.

Like many reviewers before them, they begin by noting that, “There is currently an absence of level 1 evidence (ie, from randomized controlled trials) that a program of screening and intervention for GBS reduces the risk of neonatal sepsis.” (Stephens et al 2023).

Yet hundreds of thousands of women are given antibiotics in labour, affecting their and their babies’ microbiomes. As Stephens et al (2023) show, “In the United Kingdom … it is estimated that 440 women need to receive IAP to prevent 1 case of EOD, and 3000 to prevent 1 case of EOD leading to death or severe disability.” (Stephens et al 2023).

Which approach is best?

Some people and charities have been calling for the UK to adopt universal screening, which is used in some other countries, such as the USA. However, “…the incidences of EOD in United States and United Kingdom are currently comparable.” (Stephens et al 2023). In other words, there is no evidence that the US approach is better. However, this may be partly because, as above, there is no evidence of benefit from RCTs of either approach.

Adopting universal screening would mean that even more women and babies were exposed to antibiotics:

“Moreover, modeling indicates that if universal screening was implemented in the United Kingdom, >100,000 additional women a year in the United Kingdom would receive IAP, the number needed to treat to prevent a single case of EOD would be approximately 1000, and approximately 7500 women would need to be treated to prevent 1 case of EOD leading to death or severe disability.

Consequently, in the vast majority of cases where IAP is given, the costs and risks would be incurred without clinical benefit. Moreover, widespread use of IAP and the much greater numbers needed to prevent a case of severe EOD contradict other public health initiatives that aim to reduce overall antibiotic use to address the global issue of antimicrobial resistance.

The absence of an international consensus on screening reflects an evidence base that lacks appropriately powered randomized controlled trials and a clinical scenario balancing two opposing public health priorities, namely, reducing neonatal sepsis and avoiding more widespread antimicrobial resistance.” (Stephens et al 2023).

Future options…

The paper discusses other approaches that are being explored by medical researchers, such as the possibility of in-labour testing and a GBS vaccine.

But these are not without problems either.

Stephens et al (2023) discuss how difficult it would be to test a vaccine, “…given that early-onset disease affects approximately 1 in 1000 births.” (Stephens et al 2023).

And all of these approaches still involve exposing thousands or millions of women and babies to interventions which have downsides as well as potential benefits.

Stephens K, Charnock-Jones S & Smith GCS (2023). Group B Streptococcus and the risk of perinatal morbidity and mortality following term labor. AJOG

Potential harms of GBS screening “outweigh benefits”

A review published in the British Medical Journal has considered the harms of GBS screening as well as the benefits.

And the authors have concluded that, based on current evidence, routine screening for group B streptococcus colonisation in late pregnancy should not be introduced in the UK, as the potential harms of unnecessary treatment with antibiotics may outweigh the benefits.

Farah Seedat et al (2019) explain the background to their review:

“The media and politicians regularly call for universal antenatal screening for GBS as an alternative means of selecting women for prophylaxis. Advocates point to countries across Europe and North America where screening is recommended and where reductions in early onset GBS infection have been observed. But the evidence shows that the effectiveness of screening, using established screening criteria, is uncertain and that screening has potential harms. Here, we explain why the UK National Screening Committee decided not to introduce routine screening in the UK—namely, high levels of overtreatment, unknown potential hazards from screening and intrapartum antibiotic prophylaxis treatment, and uncertain benefit.” (Seedat et al 2019).

Concern about overtreatment

There is particular concern about overtreatment and this is something of deep concern to many women and those who care for them during pregnancy and childbirth.

I have recently updated my book on this topic and, as always, sought input from women, midwives, doctors and other birth workers. Although I heard from a few people who feel strongly in favour of increased screening and treatment (often based on their own experiences of having a baby with GBS disease), the overwhelming majority of comments received were from people who feel that taking the universal screening approach is “too much”.

Some women who are offered antibiotics because they have risk factors do not want them in those circumstances either, because they are concerned about side effects and the impact of this treatment on a number of aspects of the woman’s and baby’s health and experience.

Group B Strep Explained
Group B Strep Explained; the updated second edition of a popular book which helps parents, professionals and others to understand the issues and the evidence relating to the screening and prophylactic measures offered in the hope of preventing early-onset group B strep (EOGBS) disease.

Parents’ concerns

Many of the parents who have contacted me about this topic are also aware that the chance of a baby getting GBS disease is very small, which means the chance of overtreatment is extremely high.

The new analysis shows that, “…in 2014-15, under risk based prevention, 138 933 term pregnant women were colonised with GBS, but only 350 term neonates developed early onset infection, meaning screening would have led to overtreatment of 138 583 (99.75%) women in labour.” (Seedat et al 2019).

This is of particular concern given what we know about the importance of a baby’s microbiome.

Giving antibiotics to women in labour can interfere with the development of a baby’s microbiome and, as I note in my book, “Recent research shows that not only are bacteria beneficial, but they need to be passed on to the baby during birth via its mother’s vagina and have an important part to play in future health, especially relating to the gut and digestion, but in many other areas of wellbeing as well.” (Wickham 2019).

The BMJ review also contains a couple of useful infographics which illustrate the problem of overtreatment.


Seedat F, Geppert J, Stinton C et al (2019). Universal antenatal screening for group B streptococcus may cause more harm than good. BMJ 2019; 364 :l463

The problems with GBS screening

In a 2015 debate in the British Journal of Obstetrics and Gynaecology, two respected obstetricians explained why GBS microbial screening should not be routine in pregnancy. They explained why it is so hard to measure the harms and benefits of this.

“In my view there are two main issues that hamper the assessment of harms and benefits of antenatal GBS screening.

The first is the artefact introduced by using GBS culture-positivity when assessing the outcome of the use of IAP [intrapartum antibiotic prophylaxis]. The presence of antibiotic in the baby’s circulation will inhibit the growth of the organism, despite the clinical picture of sepsis. This artefact means we remain uncertain how effective IAP really is.

The second is the huge number of women who require IAP for the small number of symptomatic neonatal infections that are likely to be prevented, with the resulting concerns about the impact that antibiotics given at the time of labour may have on the longer-term outcomes for a large number of babies.

This uncertainly about impact was highlighted by the follow-up of the ORACLE trial (Kenyon et al. Lancet 2008;376:1319–27), but extends well beyond that, with concerns that selection pressure applied to the developing microbiota may have profound long-term health consequences (Bedford Russell & Murch BJOG 2006;113:758–65; Quigley Gastroenterol Hepatol 2013;9:560–69).

Added to which, there is growing international pressure to limit the use of antibiotics in the face of increasing resistance and a lack of new antibiotic development. Being able to identify women and babies who would most benefit from antibiotics would be an important step in balancing the benefits versus harms of such a programme.”

Brocklehurst P, Steer PJ (2015).  BJOG Debate: Screening for Group B streptococcus should be routine in pregnancy. AGAINST: Current evidence does not support the introduction of microbiological screening for identifying carriers of Group B streptococcus. BJOG: An International Journal of Obstetrics and Gynaecology 122(3): 368-69.

Maternal antibiotics (e.g. for GBS) and imbalance in baby’s gut bacteria

A study published in the BJOG has given us more information about the way in which antibiotics given in labour – for example as prophylaxis for group B strep, caesarean section or because of pre-labour rupture of membranes – affect the gut microbiome of the baby. This study is welcome and timely because it confirms that intrapartum antibiotics are associated with infant gut microbiota dysbiosis (or microbial imbalance) in both vaginal and caesarean and vaginal delivery of healthy term babies.

The authors found that breastfeeding can modify some but not all of these effects.

The study included 198 healthy term babies, who were part of the longitudinal Canadian CHILD study. This might not sound a lot to readers used to research involving hundreds or thousands of women or babies, but it’s a good number for a study of this nature, which involved careful measurement of the babies’ fecal samples at 3 and 12 months. Results showed that,

“In this cohort, 21% of mothers received IAP for Group B Streptococcus prophylaxis or pre-labour rupture of membranes; another 23% received IAP for elective or emergency caesarean section (CS). Infant gut microbiota community structures at 3 months differed significantly with all IAP exposures, and differences persisted to 12 months for infants delivered by emergency CS. Taxon-specific composition also differed, with the genera Bacteroides and Parabacteroides under-represented, and Enterococcus and Clostridium over-represented at 3 months following maternal IAP. Microbiota differences were especially evident following IAP with emergency CS, with some changes (increased Clostridiales and decreased Bacteroidaceae) persisting to 12 months, particularly among non-breastfed infants.”  (Azad et al 2015).

Why does this matter?

For those who may be newer to this debate, here’s an excerpt from Group B Strep Explained, in which I wrote about the dilemma of antibiotic prophylaxis:

“No-one wants to put babies at unnecessary risk, but the irony is that one of the most significant consequences of current screening and treatment programmes which involve giving antibiotics to large numbers of labouring women whose babies are deemed to be at risk of disease (which is different from definitely having disease) may be that future generations will not have effective antibiotics even for babies who are diagnosed with actual disease.

Another potentially enormous but unquantifiable problem relates to an area that we are only just beginning to understand. I began this book by discussing the relationships between humans and bacteria, and scientists have started to use terms such as the human microbiome in order to discuss the range of microorganisms that live on and within our bodies. Recent research is suggesting that not only are bacteria beneficial, but they need to be passed on to the baby during birth via its mother’s vagina and have an important part to play in future health, especially relating to the gut and digestion, but in many other areas of wellbeing as well (Turnbaugh et al 2007, Collado et al 2012). Scientists and researchers are concerned about the potential risks to antibiotic overuse both in general (Blaser 2011) and to the baby whose mother receives antibiotics in labour (Neu 2007, Broe et al 2014). This latter concern is supported by the research showing that one of the risks of caesarean section is that these bacteria do not get passed on, which can lead to problems in babies (Grönlund et al 1999, Blaser 2011).

The problems arise because antibiotics are not particularly selective, and many beneficial bacteria will be killed by them, which may have considerable but as yet unquantified knock-on effects in both women and babies (Stokholm et al 2013). Antibiotics can create changes in women’s and babies’ gut bacteria and faecal flora which can cause gastro-intestinal problems and these changes might be permanent (Ambrose et al 1985) and they have been associated with an increased chance of postnatal yeast infection in women and babies (Dinsmoor et al 2005). We need more research into this area.” (Wickham 2019).

The authors of the current study have also noted that we need more research, as this is an important area about which we need to learn much more. But their work represents another important step in our knowledge of this area.

Azad MB, Konya T, Persaud RR et al (2015). Impact of maternal intrapartum antibiotics, method of birth and breastfeeding on gut microbiota during the first year of life: a prospective cohort study. BJOG. DOI: 10.1111/1471-0528.13601

Wickham S (2019). Group B Strep Explained. Second edition. Birthmoon Creations.

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