Final answer:
The nurse should first assist with ambulation to encourage the patient to empty her bladder, which can help correct the displacement of the uterus postpartum.
Step-by-step explanation:
The first action a nurse should take when assessing a woman 18 hours after birth whose uterus is firm, 2 finger-breadths above the umbilicus, and deviated to the right of the midline is to assist with ambulation. This deviation is often due to a full bladder which can displace the uterus and impede the process of involution, where the uterus returns to its pre-pregnancy size. Encouraging the patient to empty her bladder can help to correct the displacement. If the problem persists after the bladder has been emptied, further assessment and possibly notifying the physician may be required.