Ladythompson - most places don't do them routinely, only if you have some red flag like sugar in your urine or diabetes in the family.
It's a more heavy-weight screening test for diabetes is all - you have to fast for 12 hours, then turn up at hospital, they take your blood and you drink a sugar solution, wait for 2 hours, then more blood, then depending on the amount of glucose you either pass or fail.
I am slightly surprised I am having one now tbh, as I thought there was relatively little value in them before the 3rd trimester - but ho hum.