Ok, first of all your midwife is a loon.
Group B Strep is a bacterial infection found in about a quarter of women at any one time.
It comes and goes. The fact that you tested positive at 16 weeks means absolutely nothing.
The best time to take a test is around 37 weeks, but this is not provided by the NHS mainly due to cost. You can get the test sent to you privately, it costs around £35.
If you are still positive at 37 weeks, then yes, if you want, you can have intravenous anti-biotics during labour to prevent the baby catching GBS.
GBS can be serious in a new born baby. However you need to weigh everything up and decide for yourself whether you want the anti-biotics or not. I believe the abs gave me very bad thrush in my breasts and caused me to give up bf ds 3 years ago.
I'm now 39 weeks with number 2 and tested positive a couple of weeks ago. I want a homebirth and have decided against the abs. This has been a hard decision for me but I've weighed up the risks.
There are things you can do to help you get a negative test result, including sticking a garlic clove up your vagina overnight!
You may well have been positive last time, a quarter of women are.
Here are some links - non-scary!!
Group B Strep help site
Some natural ways to combat infection
Some more
Magic garlic!
Hope that's been helpful.