I am not pregnant, but I have been battling a post-mirena UTI for about 3 months now, most recently with the infection-causing bacteria being Group B Strep! I have never experienced anything like this, usually I have acute symptoms of UTI which pass with the first antibiotic tablet and a lot of water, and usually it's your run-of-the-mill bacteria (E. Coli) causing it. This one is hanging on for many rounds of treatments and driving me pretty much insane.
So while I'm not delivering a baby any time soon (haven't been able to have conventional sex with this blasted UTI for quite some time!) perhaps the hormonal changes Mirena put us through makes us all more susceptible to GBS.
And like SRHarder said, you shouldn't have a problem with delivery since they identified GBS, and can easily treat you for it. Best of luck with the new addition!