A guide to screening for Group B Streptococcus during pregnancy

(Aotea News, August 2009)

Dr Mark Jones
Clinical Microbiologist

The number of serious infections amongst Wellington region neonates caused by group B Streptococcus (GBS, Streptococcus agalactiae) has been on the rise recently, with tragic outcomes for some.

Tragic outcomes are, though, unnecessary as GBS is a preventable disease. If a mother is known to have GBS, antibiotics can be given at the time of delivery to protect the newborn baby.

This article gives Wellington SCL’s recommendations on how to screen pregnant women for GBS and I urge you to follow them:


Use the standard blue-capped swab (Amies swab) and, after sampling, insert into the clear jelly transport medium. Label the swab and request form appropriately, i.e. “For GBS screening”. Do not send urine for GBS screening.


The ideal time for screening is from 35 to 37 weeks of pregnancy. Any sooner carries the risk of developing late unrecognised colonisation with GBS. Any later means that the result may not get to you in time, especially if the patient goes into early labour. There is no place for “urgent screening” during early or established labour because culture results will be far too late to be of therapeutic benefit.


GBS is normally carried in the colon but spills over onto the perineum and invades the vagina. For this reason sampling the vagino-rectal area is most likely to pick up GBS carriage. Swabs may be self-taken and the patient should be encouraged to swab the low vagina first, then rub the swab over the floor of the perineum to the anus i.e. in a front-to-back direction. A high vaginal swab is not an appropriate specimen for culture (because it misses the perineum). Urine is also an inadequate specimen because a negative urine screen does not exclude perineal carriage.


It is assumed that you intend to provide intrapartum penicillin prophylaxis for all those patients whom you have screened and have found to be GBS carriers in order to prevent infection of the baby. The detection of GBS in the urine at any stage of pregnancy implies there must be exceptionally large numbers of GBS on the perineum in order to contaminate the urine and, consequently, heavy colonisation of this area is likely to persist to the time of delivery. Therefore, if a patient has been found to have GBS bacteriuria during pregnancy, intrapartum prophylaxis with penicillin is indicated regardless of screening results. (Note: we sometimes identify the presence of GBS in urine when samples have been submitted for the investigation of a possible urinary tract infection. GBS is not the cause of the UTI, but has spilled over from the perineum. We tell you about its presence so that you are aware you must give penicillin at the time of delivery).