When should a measurement of hemoglobin A1c be performed in a pregnant patient with preexisting diabetes mellitus?

The measurement of hemoglobin A1c should be performed during the first prenatal visit for a pregnant patient with preexisting diabetes mellitus because it provides essential information regarding the patient's glycemic control prior to conception and throughout early pregnancy. This timing is crucial; it allows for the assessment of the patient's risk for developing complications associated with poor glycemic control, including adverse outcomes for both the mother and the fetus, such as preeclampsia, miscarriage, and congenital anomalies.

Additionally, establishing a baseline A1c level early in pregnancy helps guide treatment decisions and adjustments in management to ensure effective control of blood glucose levels throughout the pregnancy. Regular monitoring can then be tailored based on the initial results and the patient's specific circumstances.

Measuring A1c at other times, such as in the middle of pregnancy or postpartum, may miss the critical early period when interventions are most beneficial and necessary for optimizing maternal and fetal health.