Birth Education

The GBS Test at 37 Weeks: What the Evidence Actually Says

Amy·October 1, 2026·8 min read

Group B Strep is a bacterium most of us carry without any trouble, and early GBS disease is rare. Here is an honest look at the 36–37 week swab, what the numbers really mean, how other countries handle it, and how to decide what is right for your birth.

Carrying GBS is normal. GBS disease is rare.

Group B Streptococcus (GBS) is a bacterium that lives naturally in the gut and can be found in the vagina and rectum. Around the world, roughly 18% of pregnant women carry it, and in the United States estimates range from 10% to 30%. Carrying GBS is not an infection. It usually causes no symptoms and no illness — it is simply part of the ecosystem of your body.

The reason it comes up in pregnancy is a rare complication. A newborn's immune system is brand new, and in a small number of cases a baby exposed to GBS during birth can develop early-onset GBS disease in the first week of life. That possibility is what the swab at 36–37 weeks is trying to prevent.

What the test involves

In the United States, the standard is a vaginal and rectal swab taken between 36 weeks 0 days and 37 weeks 6 days. It takes seconds and is usually painless. The sample is grown in a lab to see whether GBS is present that day.

Two clarifications worth knowing:

If GBS is found in your urine at any point in pregnancy, that is handled differently — you would be offered antibiotics in labour regardless of the swab result.
A positive swab means you are a carrier. It does not mean your baby is infected, or that anything is wrong right now.

What the numbers really mean

Here is the evidence in plain numbers, because this is the part most often left out of the conversation:

If you carry GBS and receive no antibiotics in labour, about 1–2% of babies develop early GBS disease (ACOG, 2020). Roughly half of babies pick up the bacteria from a carrier mother, and nearly all of them stay well.
Overall, early GBS disease affects about 0.2–0.5 babies per 1,000 births in high-resource countries.
Stephens and colleagues (2023) estimated that in the UK, about 440 women need antibiotics in labour to prevent one case of early GBS disease — and around 3,000 to prevent one case of death or severe disability. Modelling of universal screening put the number needed to treat near 1,000 for a case, and 7,500 for a severe case.
There has never been a randomised trial showing that screening programs save lives. Stephens et al. (2023) put it plainly: “There is currently an absence of level 1 evidence … that a program of screening and intervention for GBS reduces the risk of neonatal sepsis.”

None of this means GBS is nothing. A small number of families each year face a genuinely serious illness, and that is real. It means the decision is a true weighing of a rare but serious risk against antibiotics given to a great many women who would never have had a sick baby.

Why a woman can test positive one day and negative the next

This is one of the most important and least discussed facts about GBS. Colonisation is not a fixed state — the CDC describes it as transient, intermittent, or persistent. GBS lives in your gut, and how much of it is present in the vagina and rectum at any given moment rises and falls.

The swab is a snapshot of one single day. Birth may be four or five weeks later. Between those two moments, your body keeps shifting:

The vaginal microbiome and pH. When healthy Lactobacillus bacteria dominate, they make the environment more acidic and less welcoming to GBS. When that balance shifts, GBS becomes easier to detect.
Hormones, blood sugar, and stress. Pregnancy hormones, glucose levels, and cortisol all influence which microbes flourish. Immunity is not static either — which is why a woman may test positive, then negative, then positive again.
Recent antibiotics or other treatments. UK guidance notes that treating GBS with oral antibiotics in pregnancy does not reduce the likelihood of still carrying it at delivery — it clears it for a while and disturbs the surrounding microbiome.
Time itself. A culture taken within about five weeks of birth has a negative predictive value of 95–98%, but accuracy drifts as the gap widens. One study found a positive result at 36 weeks had a positive predictive value of only about 74% — meaning roughly a quarter of those women were no longer carrying GBS when they gave birth. Another summary found about 16% of GBS-positive women became negative by the time labour began.

Which leads to the most important number of all: even where universal screening is the policy, about 24% of early GBS cases still occur in babies whose mothers screened negative (Campbell et al., 2026). The swab is a useful clue, not a forecast of how your birth will unfold.

How the rest of the world handles it

The GBS swab is a regional practice, not a universal one.

Universal screening countries (about 60, including the US, Canada, Australia, Germany, Spain, France, and Japan) swab everyone at 35–37 weeks and offer IV antibiotics in labour to those who test positive.
Risk-factor countries (about 25, including the UK, Ireland, the Netherlands, Norway, Sweden, Finland, Iceland, and New Zealand) do not routinely screen. They offer antibiotics in labour when specific risk factors appear: GBS in the urine, a previous baby with GBS disease, preterm labour, fever in labour, or waters broken more than 18 hours. In the UK, a swab at 35–37 weeks is offered specifically to women who carried GBS in a previous pregnancy.
Rapid testing in labour. Some European units use a molecular (PCR) test during labour. The Xpert GBS test returns a result in roughly 30–60 minutes, so you know your status in the moment it matters rather than a month earlier, with reported sensitivity around 89–93%. It is not standard in the US and is rarely available at home births.

Perhaps the most striking finding: despite these completely different approaches, the rates of early GBS disease in the United States and the United Kingdom are currently comparable (Stephens et al., 2023). The UK's GBS3 trial is now testing swabbing in labour against the risk-factor approach, with results expected in 2026.

What Dr Sara Wickham's research summaries add

Sara Wickham is a midwife and researcher whose book Group B Strep Explained has become a touchstone for families weighing this decision. Her summaries of the evidence raise points worth knowing:

The evidence base itself is weak. A 2026 review by Campbell and colleagues excluded three of six systematic reviews as critically low quality; the rest rated the certainty of evidence “low to very-low” for almost every outcome.
Universal screening lowers incidence, not mortality. The headline that screening works comes from fewer GBS cases — but deaths showed no difference between universal screening and risk-based approaches.
False negatives undermine the logic. Even with screening, 24.2% of cases occurred in babies of mothers with negative cultures, compared with the 41.3% missed by risk-based care.
Antibiotic harms are underexplored. Fewer than 10% of studies followed children long-term, and a recent meta-analysis links intrapartum antibiotics with modest increases in BMI and autoimmune disease risk.
Women are barely represented. Only one study in 78 reported women's views or experiences.

Wickham's conclusion is not “never test.” It is that the decision belongs to you, made with honest numbers rather than fear.

Supporting a GBS-negative status naturally

Nothing here is proven to clear GBS, and none of it replaces the conversation about labour. What it can do is support the microbiome and immune environment in which GBS either flourishes or quietly recedes.

Probiotics. A 2022 meta-analysis (Hanson et al.) found oral Lactobacillus probiotics had a moderate effect on reducing positive GBS tests — though five of the six studies were at high risk of bias, so confidence is limited. Strains such as L. rhamnosus and L. reuteri are the most studied.
Feed the good bacteria. Fibre-rich vegetables, fermented foods, and less refined sugar all support a Lactobacillus-dominant environment.
General immune nourishment. Adequate vitamin D, iron, protein, hydration, and sleep matter for immune function, even though no supplement has been shown to turn a GBS swab negative.
Gentle vaginal care. Avoid douching, scented washes and sprays, and synthetic underwear, and let the microbiome work undisturbed. Garlic has been used traditionally — a small 2009 case series described resolving GBS vulvovaginitis — but it has not been studied in pregnancy, and inserting anything into the vagina late in pregnancy is not advisable without your midwife's guidance.
Lower the stress load. Cortisol affects immunity and the microbiome. Rest, movement, breathwork, and support are not soft extras; they are physiology.
Work with someone. If you would like herbal or nutritional support, do it with a practitioner who knows pregnancy. High-dose “natural antibiotics” can be harmful.

Reasons you might decline the test

You would rather not base a decision on a snapshot that may not reflect your status at birth.
You want to avoid a positive result leading to antibiotics in labour that you may not need.
You place a high value on protecting your baby's microbiome at birth, and on avoiding the monitoring a positive result can trigger.
The evidence does not show that screening reduces deaths, and you would rather weigh risk factors in the moment.

Reasons you might choose the test

A negative result can bring real peace of mind, and it genuinely reassures most women.
A positive result gives you time to prepare: to discuss antibiotics, plan your labour, and know your baby will be watched closely.
If you would want antibiotics in labour, the test is how you would know to receive them.
Your care provider may recommend it, and knowing your status helps you prepare for newborn observation without surprise in the first hours.

If you do test positive

A positive swab opens a conversation, not a locked door. Options include:

IV antibiotics in labour — penicillin is the standard, usually started early in labour. About four hours before birth offers the most protection, though even two hours reduces bacterial counts and the frequency of neonatal sepsis diagnoses.
Declining antibiotics while watching closely for the risk factors that matter most: fever in labour, waters broken more than 18 hours, or preterm labour.
Newborn observation — clear information about the signs of infection in a newborn, so you know exactly what to look for in the first days.
Keeping your baby skin-to-skin and breastfeeding, which supports the transfer of protective antibodies and a healthy microbiome.

Questions worth asking

What is my actual risk, given my history and this pregnancy?
What happens if I decline the swab, or decline antibiotics after a positive result?
How would antibiotics be handled in my birth setting, and does that change what I want?
If I test positive, what newborn monitoring would be recommended — and can it be done with my baby in my arms?

Every family weighs this differently, and there is not a single right answer. What matters is that the choice is yours, informed rather than frightened — and that whoever attends your birth knows and respects it. If you would like to talk through GBS, your options, or your own history, we would be glad to sit down with you.

This article is education, not medical advice. Your midwife or physician can help you apply it to your own pregnancy.