GBS is a bacteria which is present in the vagina (along with lots of other bacteria) of 20-30% of women in late pregnancy. It is potentially important because very occasionally the baby can acquire an infection with this bacteria during vaginal birth. This infection in the baby can cause serious illness, and may even be life threatening. It is important to realise that these infections are uncommon but as they are serious it is essential to have a strategy to help reduce the chance of the baby developing this type of infection.
The administration of antibiotics to the mother during labour (or once the waters have broken) is effective in preventing this infection. The tricky part is deciding which mums should receive antibiotics. There are two strategies for selecting which women to give antibiotics to. It is important to realise that neither of these strategies will prevent every infection.
Option one is to perform a vaginal swab on every woman at 35weeks gestation. If the swab shows GBS then all of those women get antibiotics in labour or when waters break (there is no benefit in treating during prior to labour).
Option two is to give antibiotics in labour only to those women who have a “risk factor” for newborn infection with GBS (there are a number of well defined risk factors).
Option one is the standard approach in USA, whilst option two is the standard approach in UK. The rate of newborn infections with GBS appears to be similar in USA and UK. There is no clear evidence which approach is better, although option two (the UK approach) means that a lot less women are given antibiotics in labour.
A recent survey of Australian obstetricians (reported in Australian & NZ Journal of Obstetrics and Gynaecology) found that about half (56%) perform a vaginal swab on all women to check for GBS whilst the other half adopt the UK approach and treat with antibiotics in labour only when a risk factor is present. At the moment there is no clear answer which approach is better.
In my practice I manage women on the basis of risk factors for GBS (option 2), but no matter which approach we take, it is not possible to prevent every infection.