Final answer:
The principal diagnosis for a maternity patient admitted at 43 weeks and having a normal delivery with vacuum extraction is O80 Encounter for full-term uncomplicated delivery.
Step-by-step explanation:
The principal diagnosis for a maternity patient admitted in labor at 43 weeks who has a normal delivery with vacuum extraction would be O80 Encounter for full-term uncomplicated delivery. Despite the prolonged pregnancy and the use of vacuum extraction, the principal diagnosis reflects an uncomplicated delivery as no complications are mentioned distinct from the labor process itself. While O48.0 Post-term pregnancy and O48.1 Prolonged pregnancy are relevant, these would be considered additional diagnoses, not the principal one. The principal diagnosis drives the overall reason for the encounter, which in this case was the delivery itself.