When does the placenta stop working in a type 1 diabetic?

Decreased blood flow. High blood sugar elves in diabetes causes damage to blood vessels. The placental vessels are both new and easy to deteriorate in this environment. As a result, blood flow diminishes and can cause problems with fetal growth.
Stop working? The placenta stops working after delivery of the infant. In a well controlled diabetic, it would be no different than a non-diabetic.