Is this test really necessary? What is it testing exactly?

My doctor wanted to give me the glucose test and I was wondering what it's supposed to test for exactly. I know it's not painful, but is it really necessary?

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    04/27/14
    The three hour glucose test is completely necessary, but a little inconvenient. What it does is it lets you/the doctors know exactly how your body is processing it's glucose and at what rate. It helps determine if you are at risk or have gestational diabetes.
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    About Ngondo Mulumba
    Born: Edison, New Jersey
    Current: Edison, New Jersey
    Birth: December 31
    On Moms.com since: Apr 27, 2014