That is the glucose screening. It checks for gestational diabetes. It is very wise idea to do it. Having gestational diabetes that you don't know about can be very dangerous.
I believe that has something to do with your glucose they are testing how well you process sugar i believe. How far along are you?
I didn't drink or eat before my test and I wonder if that affected my results. They came back as I didn't have diabetes but my mom says nor eating prior really wouldn't affect it
Oh well its hard for me to drink that thing I get really light headed & feel like throwing up & im 29 weeks