**Core Concept**
The uterus undergoes significant changes in its position and structure after delivery, transitioning from a retroperitoneal organ to a pelvic organ. This process is influenced by the relaxation of the uterine muscles and the contraction of the pelvic floor muscles.

**Why the Correct Answer is Right**
After delivery, the uterus descends from its retroperitoneal position to become a pelvic organ due to the loss of support from the uterine ligaments and the relaxation of the uterine muscles. This process is known as involution, where the uterus returns to its pre-pregnancy size and shape. The pelvic floor muscles, specifically the levator ani and coccygeus muscles, contract to provide support to the now pelvic uterus.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because the process of involution occurs after delivery, not before.
**Option B:** This option is incorrect because the uterus does not undergo a significant change in its position and structure during the menstrual cycle.
**Option C:** This option is incorrect because the uterus does not become a pelvic organ immediately after conception.

**Clinical Pearl / High-Yield Fact**
It's essential to note that the uterus may take several weeks to return to its pre-pregnancy size and shape after delivery. This process is influenced by factors such as the degree of uterine distension during pregnancy and the overall health of the patient.

**Correct Answer:** C. 6-8 weeks
The correct answer is C, as it is approximately 6-8 weeks after delivery that the uterus becomes a pelvic organ.