@MissDaphne - Yep, that's one reason you have to pee in a cup at every prenatal visit. The other is to check for protein in the urine, which is potentially even more serious because it's a sign of preeclampsia, which can be life-threatening for the mother.
Of course, gestational diabetes is pretty serious, too. Not only is it not the best thing in the world for the mom, it can cause a big, sick baby. When you read in the newspaper about someone have a fourteen or sixteen pound baby, generally the mom had poorly controlled or undiagnosed gestational diabetes. (Or, I suppose, had diabetes before she got pregnant at all.)
There are some women who want a more natural pregnancy experience (which I understand, and I love midwives) and refuse even some of these basic tests like the urine tests. Personally, I don't understand that--it's just a cup of pee, and the conditions it checks for are really serious--but to each their own, I guess.