A primigravida is posted for cesarean section due to insufficient pelvis. Pelvic inlet usually is considered to be contracted, when:
**Core Concept**
Pelvic inlet contraction is a significant factor in determining the feasibility of vaginal delivery. It is defined as a reduction in the anteroposterior diameter of the pelvic inlet, which can be measured using various indices such as the pelvic inlet anteroposterior diameter (PI-APD) or the pelvic inlet true conjugate diameter (TCD).
**Why the Correct Answer is Right**
The pelvic inlet is considered contracted when the true conjugate diameter (TCD) is less than 10.5 cm. This measurement is crucial in determining the potential for vaginal delivery, as a smaller TCD indicates a narrower pelvic inlet and increased risk of cephalopelvic disproportion. The TCD is measured from the middle of the sacral promontory to the lower margin of the pubic symphysis. A TCD of less than 10.5 cm is often associated with a higher risk of failed vaginal delivery and necessitates a cesarean section.
**Why Each Wrong Option is Incorrect**
* **Option A:** This option is not provided, but typically, the threshold for contraction of the pelvic inlet would be below 10.5 cm.
* **Option B:** This is not a standard measurement for pelvic inlet contraction, and the actual threshold for contraction is not related to this measurement.
* **Option C:** This option is not relevant to the measurement of pelvic inlet contraction, and the true conjugate diameter (TCD) is the correct measurement to consider.
* **Option D:** This option is not provided, but typically, the threshold for contraction of the pelvic inlet would be below 10.5 cm.
**Clinical Pearl / High-Yield Fact**
Remember that the true conjugate diameter (TCD) is a critical measurement in assessing the feasibility of vaginal delivery, and a TCD of less than 10.5 cm is often associated with a higher risk of failed vaginal delivery.
**Correct Answer:** C.