The thing to remember with GBS is that it is transient. You can have it one week, and test negative a few weeks later. Conversely, you can test negative, but be positive at birth. Because the test takes the lab a week or so to process, the best most people can do is to test between 35-37 weeks and hope that their result will be the same at birth (about 80% are).
Anything that you do to convert a positive test to a negative test can be temporary, and the gbs can come back later. That's why antibiotics in pregnancy don't work. If your goal is to have a negative test because of local community standards around treating that you are hoping to avoid, well that is one thing, but you can't really assume that you got rid of it if you were planning on treating. It is a tricky bug.
Like with everything, know what you are treating for, and know what you decide against as an informed choice, whatever your decision on this matter.
GBS is a big deal, despite it being relatively rare, even as compared to other neonatal infections like e-coli, is that parents' groups lobbied the government and health insurance companies to begin treating it. The impetus for prevention and treatment came from parents who had children affected by GBS and felt it was wrong that they weren't offerred a test or a way of preventing it when a way was known. It was a parental activist issue.
Now, of course, like many things, the medical community has latched on to it and now the gold standard is testing and treatment for all. No surprise there.
GBS is still very rare. It is a case where by taking the antibiotics you are reducing the chance of transmission from 1/200 (if you test positive) to 1/4000 if you take the antibiotics in labour, and 1/2000 if you take the antibiotics if you have a risk factor (fever in labour, long rupture of membranes of more than 18 hours at birth, preterm labour and birth, a gbs uti in pregnancy, a previous baby infected with gbs). About 15% of these 1/200 colonized babies will die from GBS. They typically die from meningitis, sepsis, pneumonia, or are blinded. Newborns don't process antibiotics well themselves, so they need to be given the antibiotics via their mother's placenta, and so that there are already antibiotics in their bloodstreams when they are exposed to the GBS, offering the greatest protection. Treating prophylactically for GBS means many babies are exposed to antibiotics unnecessarily.
That means that if you are positive, according to the CDC, the risk of your baby dying without antibiotics is 1/3000. If you take the antibiotics, the risk of your baby dying from GBS is 1/30,000. If you treat on risk factors, the risk of your baby dying from GBS is 1/15,000. Only each woman can decide if the exposure to the antibiotics is worth it to her to reduce the risk. The risk of an anaphylactic reaction to antibiotics in the woman is 1/10,000. The risk of an allergic, non-life-threatening reaction is 1/1000. The risk of an anaphylactic reaction in the baby is nil. Babies who become colonized will need to be admitted to the hospital and face a battery of tests, including spinal taps, an iv, and likely breastfeeding will be affected. They are profoundly ill.
There are so many things to juggle. I hate having to make up my mind. One one hand, I feel like it is a big deal over a small risk and unnecessary antibiotics for 1/199 babies, on the other hand, I feel like it is a small risk of something catastrophic happening that simple antibiotics would greatly reduce.