Final answer:
The fundus of the uterus should be within the true pelvis and not palpable by 10-12 days after childbirth, indicating that the uterus has contracted back into the lesser pelvic cavity, defined by the pelvic brim.
Step-by-step explanation:
The fundus of the uterus should be at the level of the true pelvis and not palpable by the 10th to 12th postpartum day. This is an important milestone in the involution of the uterus following childbirth. Initially, after delivery, the fundus is palpable at or near the level of the navel. However, it descends progressively into the pelvis over the subsequent days.
When the fundus is no longer palpable abdominally, it indicates that the uterus has contracted back down into the lesser pelvic cavity (true pelvis), which is the narrow, rounded space that is bounded by the pelvic brim and contains organs such as the bladder. The pelvic brim separates the true pelvis from the greater pelvis (false pelvis), and these landmarks are significant for understanding normal postpartum recovery.