"Standard everywhere?" really? Even our medwives here don't test at 20 wks!!
How about, "Did I say my mother 'had GD?' Silly me. I thought GD meant she was a great dancer." ... Although I doubt they'd go for it.
You can always say, "No, thank you." You can read up on gestational diabetes on www.hencigoer.com and Ina May, Marsden Wagner, and Sarah Buckley have good info too.