Group B Strep In Pregnancy - Part 1: What Is It And Why Does It Matter?



In the United States, a test for Group B Streptococcus (Group B Strep or GBS) is routinely given to pregnant moms between 36-37 weeks.

But why is GBS such a big concern during pregnancy? In the first part of this series we’ll explore what GBS is, why the test is recommended during pregnancy, and most importantly, the potential risks associated with passing these bacteria to your baby during birth.

What is Group B Streptococcus?

Group B Streptococcus or GBS, more specifically Streptococcus agalactiae, is one of the many bacteria that colonize the gastrointestinal and urogenital tract [1]. Around 35% of women are colonized with GBS at any given time [2]. These bugs are also commonly detected in men. 

In most  cases, GBS are not harmful and don’t cause any gastrointestinal, vaginal, or urogenital problems. Most adults will probably never experience any issues, so you may not know that you have these bacteria. However, under certain circumstances, GBS can cause infection.

Why does my provider recommend getting tested for Group B Strep in pregnancy? 

GBS in pregnancy is mostly asymptomatic. This means it will not cause any disease. In rare cases, colonization with GBS during pregnancy can cause infection, which may lead to: 

  • Bladder and urinary tract infections
  • Miscarriage
  • Stillbirth
  • Preterm birth [3]

Although not usually the case, GBS may be dangerous for a newborn baby if it is transferred from the mom during vaginal birth [3]. 

Despite being rare, GBS is the leading cause of pneumonia, meningitis, and even sepsis in newborns [1], [4].  

Depending on the time when GBS infections occur in a newborn, they are separated into two categories:

  • Early-onset infections: Symptoms  occur within the baby's first 7 days of life
  • Late-onset infections: Symptoms occur between 7-90 days of a baby’s life [1]

Early-onset infection is associated with GBS transferring from mom to baby. Late-onset infections could be due to the baby coming in contact with the surrounding environment. In this case, GBS can be transferred from hospital staff, family members, and any person that takes care of the baby [1], [3].

What is the risk of my baby getting a GBS infection?

Maternal GBS colonization during pregnancy varies between 10-30% [2]. 

It has been estimated that if mom carries GBS, and she is not treated with antibiotics during labor, the baby’s risk of becoming colonized with GBS is approximately 50% [1], [5].

But passing along GBS doesn’t mean your baby will get infected. 

Only a small percentage of babies develop symptoms and get sick. In other words, most healthy, full-term babies don’t get sick. Instead, they may develop their own GBS colonization of the skin and gut without developing infection.

According to the Centers for Disease Control and Prevention, 1 in 200 (0.5%) babies born to moms who test positive for GBS and forgo antibiotic use during delivery will develop early-onset GBS infection. This number drops to 1 in 4000 (0.025%) if the mom receives antibiotics. In 2015, 840 babies born in the US were reported to develop early-onset GBS infections. 1265 developed late-onset GBS infections [6].

While infections are rare, the consequences can be severe. 

Mortality rate from early-onset GBS infection is estimated at approximately 2% for babies birthed at term, or 2 in every 100 babies. This percentage is much higher in preterm babies and accounts for approximately 20% [6]. 

If you get a positive GBS pregnancy test and you don’t receive intrapartum antibiotics, there is a: 

  • 50% chance that your baby will be colonized 
  • 0.5% chance that your baby will develop an early-onset GBS infection

Of this 0.5%, there is a 2% mortality rate if a baby born at term develops early-onset GBS infection and a 20% mortality rate if a baby born preterm develops early-onset GBS infection.

Essentially, this means that a baby’s mortality risk from GBS is 0.001% when born at term or 0.01% when born preterm

For the math-minded: 0.5% x 2%/20% = 0.001% (at term) or 0.01% (preterm).

Are some babies more susceptible to getting GBS?

The main risk factor for early-onset GBS infection is that the mom has vaginal group B Strep during labor. 

However, some risk factors increase the chance of your baby getting infected. These include: 

  • Young maternal age, under the age of 20
  • Preterm labor, or birth before 37 weeks
  • Very low birth weight
  • Maternal fever during labor 
  • Premature rupture of membranes 
  • Prolonged rupture of membranes, 18 hours or longer [5], [7]

How do I know if I am GBS-colonized?

The American College of Obstetrics and Gynecology recommends a universal GBS pregnancy test between 36 0/7 and 37 6/7 weeks [5].

The test is easily done. Your provider will take a vaginal and a rectal swab, and send them to the laboratory to look for GBS bacteria. The test is highly accurate, but false negative results have been reported [8].

Since this routine test is given so late in pregnancy, it may be a good idea to test as early as possible - in your first trimester - to see if you are at risk. This gives you time to plan for ways to reduce or eliminate Group B Strep. Ideally, by the time your provider orders a routine GBS test at 37 weeks, you will test negative for it and won’t need antibiotics to prevent passing it to your baby.

Tiny Health’s gut and vaginal microbiome tests may reveal if you are colonized by Streptococcus agalactiae (GBS). Please note that these tests can only screen for GBS and are not meant to diagnose. 

If we detect Streptococcus agalactiae in your sample, we recommend that you discuss these results with your provider and opt to do a standard culture as described above if you want a confirmed diagnosis.

Can I skip the GBS pregnancy test? 

GBS testing during pregnancy is routinely recommended in the US, but it is not mandatory. Many other countries do not screen for GBS. 

The UK, Australia, New Zealand, and many countries in Europe follow a risk-based approach when it comes to GBS. This means that the GBS test is not recommended for all pregnancies. But women with complications during delivery, such as prolonged labor, fever, and premature birth will most likely get antibiotics. 

We recommend doing what feels right for you. If doing a test will give you peace of mind, go ahead and do the test. If you want to skip the test, you have the right to refuse it.

In the second part of this series on Group B Strep in pregnancy we’ll delve into what you can do if you test positive for GBS𑁋whether it is towards the end of your pregnancy or earlier through a gut or vaginal microbiome test.

>> Group B Strep in pregnancy  - Part 2: what to do if you test positive


[1] J. A. Morgan, N. Zafar, and D. B. Cooper, “Group B Streptococcus And Pregnancy,” in StatPearls, Treasure Island (FL): StatPearls Publishing, 2022. Accessed: Jan. 20, 2022. [Online]. Available:

[2] N. J. Russell et al., “Maternal Colonization With Group B Streptococcus and Serotype Distribution Worldwide: Systematic Review and Meta-analyses,” Clin. Infect. Dis. Off. Publ. Infect. Dis. Soc. Am., vol. 65, no. suppl_2, pp. S100–S111, Nov. 2017, doi: 10.1093/cid/cix658.

[3] A. C. Seale et al., “Estimates of the Burden of Group B Streptococcal Disease Worldwide for Pregnant Women, Stillbirths, and Children,” Clin. Infect. Dis. Off. Publ. Infect. Dis. Soc. Am., vol. 65, no. suppl_2, pp. S200–S219, Nov. 2017, doi: 10.1093/cid/cix664.

[4] L. Madrid et al., “Infant Group B Streptococcal Disease Incidence and Serotypes Worldwide: Systematic Review and Meta-analyses,” Clin. Infect. Dis. Off. Publ. Infect. Dis. Soc. Am., vol. 65, no. suppl_2, pp. S160–S172, Nov. 2017, doi: 10.1093/cid/cix656.

[5] “Prevention of Group B Streptococcal Early-Onset Disease in Newborns: ACOG Committee Opinion Summary, Number 797,” Obstet. Gynecol., vol. 135, no. 2, pp. 489–492, Feb. 2020, doi: 10.1097/AOG.0000000000003669.

[6] S. A. Nanduri et al., “Epidemiology of Invasive Early-Onset and Late-Onset Group B Streptococcal Disease in the United States, 2006 to 2015: Multistate Laboratory and Population-Based Surveillance,” JAMA Pediatr., vol. 173, no. 3, pp. 224–233, Mar. 2019, doi: 10.1001/jamapediatrics.2018.4826.

[7] N. J. Russell et al., “Risk of Early-Onset Neonatal Group B Streptococcal Disease With Maternal Colonization Worldwide: Systematic Review and Meta-analyses,” Clin. Infect. Dis. Off. Publ. Infect. Dis. Soc. Am., vol. 65, no. suppl_2, pp. S152–S159, Nov. 2017, doi: 10.1093/cid/cix655.

[8] L. Filkins, J. R. Hauser, B. Robinson-Dunn, R. Tibbetts, B. L. Boyanton, and P. Revell, “American Society for Microbiology Provides 2020 Guidelines for Detection and Identification of Group B Streptococcus,” J. Clin. Microbiol., vol. 59, no. 1, pp. e01230-20, Dec. 2020, doi: 10.1128/JCM.01230-20.