When to test for GBS?
If you’re testing for group B Strep carriage, it’s usually recommended between 35 and 37 weeks of pregnancy.
The result of a test specifically for group B Strep carriage is highly predictive of group B Strep carriage status for the next 5 weeks, so it’s best done in the last 5 weeks of pregnancy.
Tests before 35 weeks of pregnancy – sometimes too early
If the test is taken earlier than 35 weeks, you are more likely to go into labour more than 5 weeks later and so your carriage status is more likely to have changed by then (ie you may either pick up group B Strep carriage or lose it in the intervening weeks).
A positive result from a test before 35 weeks of pregnancy means you should be offered intravenous antibiotics when you go into labour, though you may not still be carrying the bacteria then. A negative result from a test before 35 weeks of pregnancy means that you will not be offered the intravenous antibiotics in labour (unless other risk factors develop), although you may acquire carriage by the time you go into labour.
The Royal College of Obstetricians & Gynaecologists recommends that all women in confirmed preterm labour (before 37 weeks of pregnancy) should be given intravenous antibiotics whether or not they are known to carry group B Strep.
Tests after 37 weeks of pregnancy – sometimes too late
Tests taken later than 37 weeks will give you a very good indication as to your carriage status for the next 5 weeks, during which time you are most likely to give birth. However, there is a chance that your results will not be ready when you go into labour, and your baby may arrive before they’re available.
Tests between 35-37 weeks of pregnancy – usually just right
The 35-37 weeks ‘window’ is suggested as being usually the best time to test as the result will be highly predictive of whether you will be carrying group B Strep when you are most likely to go into labour (during the next 5 weeks). However, it won’t suit everyone, not least those who give birth later than anticipated.