why do i need a gtt test at 28 weeks?

[soppypreggyloon]

my mw told me i needed one last time i saw her but as we'd been waiting for 40mins and ds was very restless i forgot to ask why.

i didn't have one with ds
i'm not overweight
i have no history of diabetes

ds was big ish - 8lb10
i had a emcs

i don't mind doing one if i have to but i'd like to know why.
any ideas?

I'm having an elcs in any case so I know that won't change anything. More scans would be a bit of a pest getting to hospital/ organising child care etc but are quite reassuring I guess.
I'd rather know in advance if there was a chance of dc2 being unwell due to gd than not.