Why do they test so late in pregnancy for Gestational Diabetes?

Can you answer Brittany’s question about Diabetes?:

I only have 14 weeks left in pregnancy and my doctors are saying they want to do the test for Gestational Diabetes. Why it is it important to have this test? The babys body is developed already, so why dont they do this test a lot earlier in pregnancy? What happends if I say no to the test? Anything bad happends?

Hyperglycemia Signs