Final answer:
The highest priority nursing intervention on a client's first postpartum day is fundal massage and assessment.
Step-by-step explanation:
The highest priority nursing intervention in this case would be fundal massage and assessment. After childbirth, the uterus needs to contract and shrink back to its pre-pregnancy size, and fundal massage helps stimulate this process. A boggy uterus, which is soft and enlarged, can lead to postpartum hemorrhage. By gently massaging and assessing the fundus, the nurse can promote uterine contraction and check for signs of excessive bleeding or retained placental fragments.