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Group B Streptococcus (GBS)

Group B Streptococcus (GBS)


Group B Streptococcus is a ‘gram-positive’ bacteria which naturally occurs in the genital and gastrointestinal tracts of people of all ages. It predominantly resides in the intestinal tract and migrates down to the rectum, vagina and urinary tract.

In pregnancy, Group B Streptococcus (GBS) colonization is not uncommon (research shows approximately 10-30%). However, colonization does not indicate an infection, and most people with GBS have no infection or symptoms.

When GBS does cause an infection, it may cause urinary tract infections and/or infection of the newborn.

In newborns, Group B Streptococcus can be a major cause of infection either in the brain (meningitis), lungs (pneumonia) or infection of the blood (sepsis).  These infections, although rare, can have serious and life-threatening complications.

Risk factors for GBS

  • Premature delivery (before 37 weeks gestation)
  • First pregnancies
  • Prolonged membrane rupture (time between water breaking and birth)
  • Fever during labour
  • Previously diagnosed as GBS positive
  • Infection of the uterus (called chorioamnionitis)


Testing for GBS

Testing for Group B Streptococcus is done by swabbing the vagina and/or rectum with a Q-tip and then allowing 24-48hrs to see if GBS cultures grow. This test is performed between 35-37 weeks of pregnancy with aims to diagnose GBS status prior to labour.

Adults may also test positive for GBS temporarily or persistently, which is why testing in early pregnancy isn’t necessary or completely reliable. It is also worth noting testing isn’t considered completely reliable overall, and that false negatives can occur.

How common is GBS infection of the newborn?

There are two main types of GBS infection in newborns – early onset (occurring within the first 7 days following birth) and late onset (diagnosis up until age 90 days).

Early onset GBS is the most common, with symptoms typically appearing within the first 24-48hrs, leaving researchers to suggest early GBS infection actually begins before birth.

Early infection is caused by vertical transmission, i.e. directly from mother to baby – usually after the waters have broken. The Group B Streptococcus can migrate into the amniotic sac once the waters have broken and may cause bacterial infection in the newborn.

Current recommendations

In the early 1990’s, evidence began to emerge supporting the risks of GBS infection to newborns. By 1994, the American Congress of Obstetricians and Gynaecologists recommended screening all pregnant women for GBS and treating all GBS positive women with intravenous antibiotics in labour.

Following this recommendation, there was a significant decrease in early onset GBS infections in newborns. Statistics in the US reflected a decrease of early onset GBS infections from 1.7:1000 births to 0.25:1000 births during this time.

In 2010, the CDC (Centres for Disease Control and Prevention) published their universal recommendations: Women should be tested for GBS between 35-37 weeks of pregnancy, and antibiotics administered during labour for women considered at risk.

If a mother is GBS positive and is not treated with antibiotics in labour, the risk of their baby becoming GBS colonized is approximately 50%, and the risk of developing a life-threatening GBS infection is <2%. (Due to the fact colonization is not the same as GBS infection)

How will my baby be impacted by antibiotics in labour?

Since 2014 there have been several studies into the effects of antibiotic use during labour and their effect on infant’s microbiome.

Research was collected by comparing stool samples at different ages ranging from 3 days old up until 12 months of age. Researchers concluded the infant microbiome was impacted when their mothers had antibiotics in labour (commonly for GBS) as opposed to the infants of mothers who did not.

These changes to the microbiome were an increase in non-beneficial bacteria and a decrease in their natural, beneficial bacteria. This is because antibiotics given to fight gram-positive bacteria (such as GBS) can lead to the overabundance of gram-negative bacteria.

Results vary between studies, with some research suggesting the infant microbiome recovering by 4-8 weeks, and other studies suggesting recovery requiring 3 months or more.

There was also significant research to support the benefits of breastfeeding to help infant microbiome recovery, with results suggesting infants of caesarean deliveries who did not breastfeed experienced microbiome changes until 12 months of age.

So, what’s the bottom line?

  • Evidence supports the use of screening in pregnancy and treatment of all GBS positive women in labour to lower the rates of GBS infections in newborns
  • Evidence suggests antibiotics negatively impact the infant microbiome, however for most infants these impacts are temporary and can be reduced by breastfeeding
  • There are both positives and negatives to antibiotic use in labour – it is best to discuss these with your healthcare provider


Written by Keryn Thompson RM & IBCLC (L-301766)


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