Final answer:
The nurse should first notify the healthcare provider when a fetal heart rate of 59 beats/min is detected, as it indicates possible fetal distress and the need for urgent intervention.
Step-by-step explanation:
When a nurse assesses a fetal heart rate (FHR) of 59 beats/min during labor, the first action to take is notifying the healthcare provider. A heart rate of less than 60 beats/min in the fetus can indicate fetal distress, which may require urgent intervention such as the administration of oxygen to the mother, a change in maternal position, or preparation for an emergency delivery if the situation does not improve. Immediate communication with the healthcare provider is critical for determining the appropriate next steps and potentially initiating emergency procedures such as an emergency cesarean section if necessary.