Oral Glucose Tolerance Test (OGTT) in Early Pregnancy: Safety & Timing

Question
I'm writing in the hope that you will answer a question regarding oral
glucose tolerance testing during pregnancy. It is my understanding that
these tests are normally given at the end of the second trimester of
pregnancy to screen for GD. I'm wondering under what circumstances, if
any, are these tests given in the first trimester of pregnancy, and if
they are, whether they are considered safe given the association of high
blood glucose levels with the potential for fetal harm.

Thank you!

Answer
We usually test for gestational diabetes at 28 weeks gestation with a one hour glucose tolerance test. If the value is greater than 40, we send the patient for a three hour GTT to get a more accurate test. If there are two abnormal values in the GTT, the dianosis is made. If the patient is a known diabetic, she does not need the test at 28 weeks. If the patient shows signs of diabetes early in pregnahcy (perhaps at the first visit) we would send her for a three hour GTT right away (if she is obese, or has symptoms (thirst, polyurea, polyphagia), or if she is spilling sugar on a dipstick. The short test will not cause fetal harm.