Final answer:
The gestational age that defines a postterm pregnancy is 41 completed weeks.
Step-by-step explanation:
The gestational age that defines a postterm pregnancy is C. 41 completed weeks. A full-term pregnancy typically lasts for approximately 40 weeks from the first day of the last menstrual period (LMP). Obstetricians set the due date as 284 days (around 40.5 weeks) from the LMP.
Postterm pregnancy is defined as reaching 41 completed weeks or more. It is important to closely monitor the fetus and the pregnant person in postterm pregnancies to ensure their well-being.