[Clinical] 26w GA Pregnant woman with fasting glucose of 277: Overt or GDM?

Confused MS here. Attending said overt;

I tried to find any text that confirms this, but UpToDate says that:

If a patient in early pregnancy (before significant insulin resistance) meets standard ADA criteria for diabetes, she is assumed to have had the disorder prior to the pregnancy, but there is no way to determine at what gestational age this would no longer be true. (See "Clinical presentation and diagnosis of diabetes mellitus in adults".)

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A fellow ObGyn resident told me that I could differentiate overt DM from GDM using ADA's criteria, i.e., at any point during pregnancy, fasting glucose >126 diagnoses overt DM. But that doesn't make any sense to me because, as UpToDate says, it's impossible to tell if the patient has insulin resistance due to max hPL levels (24-28w) or due to another cause (overt). Is the final answer just wait until the postpartum period to reassess? Thanks for helping!