I have dilated ureters, but it wasn't picked up until I was pregnant with Darcy.
I have had lots of UTIs/cyctitis and I think people with dilated ureters are often predisposed to urethral reflux (urine going back up to the kidney). For me pregnancy was a killer and my kidney function has never recovered.
My dilations are due to obstruction (the valves in my bladder are dodgy) as are many cases. I'm pretty sure with newborns they will look at the efficacy of unblocking while they are little so that potential problems are minimised.