Gestational Diabetes

For those Mummies with GDM, I’m just curious on what instance that our Dr will determine that one need to be on metformin or insulin when we no longer able to control our sugar level through diet? I heard that when our sugar level spike to above 7 or 8 continuously after meals? Is this true? Is it true also when we are at our last stretch of the pregnancy and the body not able to break down the sugar more efficiently as it was before could be 1 of the factor?