Final answer:
Fasting plasma glucose (FPG) is usually normal in true GDM because insulin production increases during pregnancy to compensate for insulin resistance. This means that while fasting blood glucose levels may remain in control, the inability to handle postprandial glucose loads can reveal glucose intolerance indicative of GDM.
Step-by-step explanation:
The question 'Why is FPG usually normal in true GDM (Gestational Diabetes Mellitus)?' is related to the physiology of blood glucose regulation during pregnancy. Option (b) 'Fasting glucose is regulated by insulin, which is often increased in GDM' correctly describes this situation. During pregnancy, especially in the context of GDM, the fasting plasma glucose (FPG) can often be normal because insulin production is increased to compensate for the insulin resistance that characterizes GDM. This is in contrast to situations later in the day when the woman's capacity to produce insulin may not be sufficient to cope with the postprandial glucose load, leading to abnormal glucose tolerance tests.
GDM is characterized by glucose intolerance of varying degrees with onset or first recognition during pregnancy. Gestational diabetes usually manifests in the second or third trimester, a time when fetal demands for energy nutrients are very high. These increased demands result in insulin resistance to ensure an ample supply of glucose for the fetus. The beta cells of the pancreas in pregnant women may be able to compensate for this resistance by increasing insulin production, enabling normal fasting blood glucose levels to be maintained, though this compensatory mechanism may fail under the stress of a glucose load as seen in oral glucose tolerance tests.