Final answer:
The most important action for the nurse to take is to assess for the amount and character of the lochia.
Step-by-step explanation:
When a nurse notes that the fundus is approximately 1 cm above the umbilicus 12 hours after delivery of a 3,200 g infant, the most important action for the nurse to take is to assess for the amount and character of the lochia. Lochia is the postpartum vaginal discharge that consists of uterine lining cells, erythrocytes, leukocytes, and other debris. By assessing the lochia, the nurse can monitor for any signs of excessive bleeding or infection, which are important to identify and address promptly to ensure the mother's well-being.