I started NSTs around 34 weeks. I think it is pointless to start them any sooner because I believe the whole point of them is to make sure your baby is getting the right oxygen and nutrients through the placenta (and will therefore not be in distress) which means the placenta is not breaking down yet. If it is breaking down and the baby is in distress, they would induce you early/ do a c-section. But at 28 weeks, there is no way the placenta is breaking down and definitely no way they will take your baby out yet. If you went into labor between 28-32 weeks, they would give you drugs or do other procedures to stop it.
NSTs certainly won't hurt anything even if you started at week 4 or something but in my opinion they won't do any good either and will just waste your time. Ask your OB what the point is of NSTs and what the point is of starting them so early in your pregnancy.
If you haven't done an NST before, they just strap two belts with little round monitors around your belly- a fetal heart-rate monitor, and a contractions monitor (in case you're having any- you probably aren't yet) and lay there for 20-40 minutes. They may have you push a button every time the baby moves. (Seriously, they don't have a monitor that can pick that up automatically?)
Congratulation on your first pregnancy (and great A1C!) and good luck!