Final answer:
The nurse should first assess the fundus to check the tone of the uterus and to help control postpartum bleeding when a perineal pad is soaked soon after vaginal delivery.
Step-by-step explanation:
If a nurse observes that a perineal pad is soaked with blood and there is blood underneath the client's buttocks six hours after a vaginal delivery, the first action the nurse should perform is to assess the fundus. Checking the fundus can provide immediate information about the tone of the uterus and whether it is contracting properly to control bleeding. Once the fundus is assessed and if it is found to be boggy, measures such as massaging the fundus may be taken to firm it up and help prevent further bleeding. After addressing the fundus, the nurse would typically monitor the patient's blood pressure and notify the healthcare provider of the situation. Administering oxygen would be an appropriate intervention if the patient exhibits signs of hemodynamic instability or oxygen deprivation.