I was underweight with my first pregnancy and
also didn't eat a lot of sweets, and
I ate healthy and
still failed the one hour and
3 hour glucose tests. Not horribly, but enough to send me to the nutritionist. i also was exercising and active.
Up until pregnancy I'd had bouts of hypoglycemia, where my sugar would plummet if I hadn't eaten in a while, and honestly, having higher blood sugar was a welcome relief.