Group B streptococcus (Streptococcus agalactiae; GBS) is a Gram positive bacterium that is one of the leading causes of infectious neonatal morbidity and mortality. The bacteria can colonise (be present without causing disease) the vaginal and gastrointestinal tracts in healthy women, and it has been estimated that approximately 14% of all women in the UK are carriers (Colbourn et al. 2007). During labour and birth, babies may come into contact with the bacteria in the birth canal and may themselves become colonised.

Although most babies are unaffected by GBS colonisation, a small number may develop clinical infection, known as early‑onset GBS infection. Clinical infection typically happens within the first 12 hours of the baby's life, and the baby can become symptomatic between birth and 7 days of life. It is estimated that 1 in 2000 babies born in the UK and Ireland develop early onset GBS infection (UK National Screening Committee 2012). Consequently, approximately 340 of the 680,000 babies born in the UK each year are likely to develop early‑onset GBS infection. The reasons why only some babies who are colonised with group B streptococcus go on to develop early‑onset GBS infection are not well understood.

Most babies who develop a clinical GBS infection are successfully treated and make a full recovery. However, despite good medical care the infection can go on to cause life‑threatening complications such as septicaemia, pneumonia and meningitis. Mortality among babies with early‑onset clinical GBS infection is 10–30%, with the highest risk in premature babies (Mueller et al. 2014). One in 5 babies who survive the infection will be permanently affected and may have problems such as cerebral palsy, blindness, deafness and serious learning difficulties (NHS Choices 2013). For every woman with GBS colonisation during birth, the risk to their baby of neonatal death from early‑onset GBS neonatal sepsis is 0.03% (UK National Screening Committee 2012).

Currently, the Royal College of Obstetricians and Gynaecologists (RCOG) does not recommend routine screening or testing for GBS colonisation in pregnant women in the UK, because the clinical and cost effectiveness of this strategy remains unclear (RCOG 2012). Similarly, the National Screening Committee does not recommend routine testing or screening for GBS at any stage during pregnancy (UK National Screening Committee 2012). In addition, NICE guidelines on antibiotics for early-onset neonatal infection and intrapartum care do not recommend selective testing for GBS in women considered to be at increased risk of GBS transmission.

Intrapartum antibiotic prophylaxis of pregnant women is believed to reduce the incidence of early onset GBS infection in babies, although there is a lack of robust evidence to demonstrate this (Ohlsson and Shah, 2014). It is thought that the optimal antibiotic regimen to prevent GBS transmission is 2–4 hours of intrapartum antibiotic prophylaxis, for example, with benzylpenicillin or clindamycin (RCOG 2012).

GBS may be detected incidentally during the early stages of pregnancy from a routine urine test indicating signs of an infection (bacteriuria). Microbiological techniques such as standard direct plating or enriched culture medium plates are then used to identify the bacteria.