Another Group B strep question!

[bananastew]

If you're positive for GBS would it show anytime throughout the pregnancy? I had a high swab at 34 weeks and have been advised I may need another one now 37 weeks.

I'm not sure of the results from 3 weeks ago but would be able to find out. If that was negative could I have developed GBS since??

To begin with, a high swab test isn't wildly accurate while it has a low false positive rate it has a very high false negative rate and only picks up GBS in about 50% of cases where it's there. So if you have a negative high swab test at 34 weeks not only could you have GBS at 40 weeks, you could have GBS at 34 weeks in spite of the negative result. The ECM (Enriched Culture Medium) test is far far more accurate, although is only available on the NHS in a couple of areas you can get it done privately at relatively low cost, though (there are links on the GBS Support website).

In general, though, the recommendation is to have GBS testing between 35 and 37 weeks, because studies show that if an ECM test is done within 5 weeks of delivery, a negative result is 96% predictive of not carrying GBS at delivery (i.e. 4% of women who originally test negative will acquire GBS between the test and giving birth) and a positive result is 87% predictive of carrying GBS at delivery (i.e. 13% of women who originally test positive will lose GBS between performing the test and giving birth). Not sure if the equivalent figures have been run for high swab tests. Obviously a test done at 34 weeks isn't worthless, but it's less reliable.

Thanks! I'll have another chat with my MW and see what she says!