I started to look at this with my second pregnancy. This is something that any woman should look at before she has the glucose tolerance test in her second trimester.
The glucose tolerance test was not even started with the idea that high sugar levels at the end of pregnancy was harmful. It started with research to see if women who had elevated levels in pregnancy had an increased risk of developing Type II diabetes later in life. This was found to be true, but from that somehow gestational diabetes was born.
Diabetes can be harmful during pregnancy. The most critical time for the baby is during the first trimester, though. This is when most eyesight problems or birth defects occur. The glucose tolerance test is administered in the second trimester missing this critical period. If diabetes is a major concern it would be better to get tested for it before getting pregnant.
This is a link to an article that I think explains both the pros and cons to the glucose tolerance test. I highly encourage reading it.