Informed Choices: Group B Strep (GBS) Swab

I’m skipping a bit “ahead” on the pregnancy informed choices trajectory because I just did my GBS swab and it’s fresh on my mind.

Disclaimer: This blog is not medical advice. This blog is for informational purposes only. Please seek advice from your trusted healthcare provider.

First of all – what is Group B Strep?

Group B Streptococcus (aka Group B Strep or GBS) is a bacteria that can be found various places in our bodies including the reproductive and digestive systems. It is not constantly present but can come and go (similar to other bacteria we have in these areas!). You often won’t know if you have or “carry” GBS because it doesn’t usually cause problems.

In pregnant women, the carrier rate for GBS is 1 in 3 to 4 women in the USA. A carrier may develop an active GBS infection, so this is the risk that the carrier status presents.

Sometimes, GBS infections can be symptomatic in pregnancy or post delivery. UTIs or uterine infections after pregnancy may occur if this is the case, as well as infections of the amniotic fluid.

When and why is this test offered to me in pregnancy?

Why do we test for it in pregnancy if it’s often present and not generally an issue? Firstly, an active GBS infection during pregnancy can lead to preterm labor or birth, and other risks such as stillbirth if baby becomes infected during pregnancy.

Because it can be found in the vagina as well as the rectum during pregnancy, it can pass to baby during birth. This is when there is a risk for it becoming more problematic. Although passing it is considered rare (1 to 2 out of every 100 babies of infected mothers or birthing persons), antibiotics are usually recommended to further reduce the risk of complications related to this.

Complications related to GBS in a newborn include: meningitis, pneumonia, sepsis or in some cases death, within 24-48 hours of the birth (this is considered early-onset). There can also be some late symptoms (late-onset) that occur when baby is a few weeks or even a few months old. This is related to post-delivery contact with a positive parent or mother (or anyone!). Symptoms of late-onset disease may include: irritability, fever, decreased energy and issues with feeding.

The GBS swab test is usually offered between 35-37 weeks of pregnancy (or 36+0 and 37+6 depending on the source and where you are) as it should be done within ~4 weeks of the due date or expected delivery so the results remains current and accurate (see more on accuracy below). Sometimes it may need to be done again if it is done early and mom goes post-dates (after due date). It is a vaginal and rectal swab test the mother can do herself if instructed (I love this doctor’s view on changing practice to allow patients to perform their own vaginal swabs for things like vaginitis, etc.). Results are available within a few days.

I tested positive but I’m having a C-section, what now?

Since the bacteria is passed via the vagina and rectum, you usually would not need antibiotics if you’re having a planned C-section. The risk of passing any infection on would be much lower. However, it is recommended you still be tested in case labor begins before your scheduled date. They may also be recommended if your amniotic sac has broken.

What are the risks and the benefits of doing or not doing this swab? What about the antibiotics?

As this is just a vaginal swab, there are virtually no risks to doing the test. One thing I have mentioned before is that these swabs (and swabs for yeast and other vaginal cultures) can be done by the mother to reduce trauma or anxiety associated with having a provider do it. There are also other benefits to self-swabbing – you can read about some in this great post shared by an MD who changed their practice to support self-swabs or “selfies”.

Benefits of doing the test include the identification of GBS carriers and prevention of possible infection and related complications. Again, although these are rare – it is not rare to be a carrier. You simply cannot predict whether you will develop an infection or not – so doing the swab at least allows you to be prepared and to make more informed decisions.

How accurate is the test?

The test is considered relatively accurate. A good resource to review this aspect specifically is Evidence Based Birth.

For testing at 35-36 weeks, a study found that 91% of women were still GBS negative at birth. Similarly, 84% were still GBS positive at birth when compared with their 35-36 week test results. Notably, that is still 16% of people who received (or may have received) antibiotics that were not necessary. If you have concerns about this, please speak to your healthcare provider.

Studies have been ongoing for rapid testing during labor for GBS, since some positive cases are missed within the current testing window & to reduce the use of unnecessary antibiotics and the development of antibiotic resistance – you can read more on this on Evidence Based Birth.

What are the risks and benefits of the antibiotic treatment?

Some studies show a significant reduction in newborn risk of infection with GBS when mom receives IV (intravenous) antibiotics during labor. However, these results have been brought into question – again, I refer you to the excellent review by Evidence Based Birth on this topic for a more detailed breakdown of the meta-analysis that rendered and critiqued these results.

We do know that penicillin, the first antibiotic of choice for GBS treatment (barring any allergies which I will discuss in a moment), is effective in newborn GBS infection prevention via placental transfer. We have also seen from large studies in the US that the swabbing + treatment for positives is effective in reducing newborn infection rates (and therefore, its related complications discussed above) when compared to the risk factor based approach (ie. only offering treatment to women who have particular risk factors for GBS).

Antibiotic resistance has not been a concern for penicillin GBS treatment, though a study done in 2016 did find resistance in Italy. This is obviously a topic that needs more research going forward. Based on the results shared by Evidence Based Birth above re: positive tests at 35-36 weeks no longer being positive at delivery, perhaps need for testing early in labor is needed for only positive cases – this is just a shot in the dark by me, I’m not an expert nor do I claim to be.

Risks of antibiotic treatment for GBS remain consistent with risks that exist when taking antibiotics in general. There is a risk for allergy or anaphylaxis with penicillin. If you have a known penicillin allergy, let your care provider know asap. When you start prenatal care, questions about allergies should be asked, and they should also be repeated at subsequent visits. You should also have an opportunity to record these on hospital pre-admission forms. Skin testing may be recommended if your allergy is not serious (anaphylactic) or confirmed (many people think they are allergic and end up not being so!). Non-serious reactions may be offered an alternative to penicillin, while anaphylactic ones will require testing to determine the best option for treatment. You can read about current evidence around alternative antibiotics here.

It is possible that you won’t know if you are allergic to penicillin and have a serious reaction – however, this is rare.

Microbiome is a hot word lately and antibiotics can impact a newborns microbiome. (Don’t know what this word is? Click here). Evidence shows, in regards to this risk, that a newborns microbiome recovers quickly and this recovery is notably aided by breastfeeding or breast milk. Similarly, yeast infections often occur after antibiotic use and this has been shown to impact breastfeeding if the yeast infection occurs in the infants mouth (thrush) or on the mothers breasts, for example. Yeast infection prevention can be done to help avoid this, and treatment can be sought. I spoke about yeast infection prevention & treatment here.

If you are looking for more evidence-based information on the GBS swab – speak to your care provider, your #1 resource for your health information! You can also check out some the reliable alternative resources below (to be used in addition to discussion with your provider).

My Personal Experience

I try to include a section like this where applicable, because I appreciate the value of sharing our stories (as well as the comfort it can provide). I personally have done two GBS swabs – one in each pregnancy, around 36-37 weeks both times. I only ever had to do one per pregnancy because I fell within the “window” of time. I did both myself – as I mentioned above, this should be an option for you if you want. Please speak to your provider about this if you have concerns.

My results took about 2 business days. Both swabs have been negative for me – I don’t know if that means I’m “not” a carrier, or at least haven’t been during pregnancy. I therefore have no experience to add regarding the antibiotics. However – when I start up the podcast again in the coming weeks, I hope to bring this series on there and discuss these things more with both experts & those who have experienced these first hand. So stay tuned for that!

In terms of the decision-making – this one was a no-brainer for me. The swab is just a swab, and I know the occurrence is rare but I figured it was something in my power to prevent. A lot of things in pregnancy and birth are “outside” of our will – we cannot change them or impact them much, such as when baby chooses to arrive or how a labor will go down to every nitty gritty detail. This was something I could control – I could do the swab, know my result and act accordingly if needed. That gives me comfort so I decided to pursue it.

Additional Resources

American College of Obstetricians and Gynecologists – GBS and Pregnancy

UpToDate – GBS and Pregnancy

Evidence Based Birth – GBS in Pregnancy


One response to “Informed Choices: Group B Strep (GBS) Swab”

  1. […] rewind from my last post on the GBS swab, which happens later in pregnancy, to glucose screening! This topic comes at an appropriate time, […]


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