The most important pelvic inlet diameter through which of the foetal head passes during birth:
A. Diagonal conjugate
B. Anatomical conjugate
C. Obstetrical conjugate
D. Oblique diameter
Correct Answer: C. Obstetrical conjugate
Explanation
Most important pelvic inlet diameter:
👉 Obstetrical conjugate
Explanation:
- The pelvic inlet is the entrance of the birth canal.
- The fetal head must pass through the smallest anteroposterior diameter of the pelvic inlet.
- This critical and limiting diameter is the obstetrical conjugate.
Why the obstetrical conjugate is most important:
- It represents the shortest anteroposterior distance between:
- The posterior surface of the symphysis pubis
- The promontory of the sacrum
- It determines whether the fetal head can engage in the pelvis.
- If this diameter is reduced, obstructed labour may occur.
Key points:
- Average length: ~ 10–10.5 cm
- Cannot be measured directly during pelvic examination.
- It is estimated from the diagonal conjugate.
Comparison with other inlet diameters:
- Anatomical conjugate: Longer, but not used for labour assessment.
- Diagonal conjugate: Clinically measurable, used to calculate obstetrical conjugate.
- Transverse diameter: Wide, rarely limiting.
✅ Therefore, the obstetrical conjugate is the most important pelvic inlet diameter through which the fetal head passes during birth.
Other Options Details
Here is a clear obstetrics-oriented explanation of each pelvic diameter:
A. Diagonal Conjugate
- Definition:
The distance from the lower border of the symphysis pubis to the sacral promontory. - Clinical importance:
It is the only anteroposterior diameter that can be measured clinically during vaginal examination. - Average length:
About 12–12.5 cm - Use:
Helps to estimate the obstetrical conjugate (by subtracting ~1.5–2 cm).
B. Anatomical Conjugate (True Conjugate)
- Definition:
The distance from the upper border of the symphysis pubis to the sacral promontory. - Average length:
About 11 cm - Clinical importance:
It represents the bony pelvic inlet, but cannot be measured clinically. - Use:
Mainly of anatomical interest, not directly useful in labour.
C. Obstetrical Conjugate
- Definition:
The distance from the posterior surface of the symphysis pubis to the sacral promontory. - Average length:
About 10–10.5 cm - Clinical importance:
It is the most important pelvic diameter, as it is the narrowest anteroposterior diameter of the pelvic inlet. - Use:
Determines whether the fetal head can enter the pelvis. - Measurement:
Cannot be measured directly; calculated from diagonal conjugate.
D. Oblique Diameter
- Definition:
The distance from the sacroiliac joint on one side to the iliopubic eminence on the opposite side. - Average length:
About 12 cm - Clinical importance:
Important for engagement and rotation of the fetal head during labour. - Presence:
Two oblique diameters (right and left).
Summary Table
Diameter Length (cm) Measurable Clinically Importance Diagonal conjugate 12–12.5 Yes Estimates obstetrical conjugate Anatomical conjugate ~11 No Anatomical reference Obstetrical conjugate 10–10.5 No Most important for labour Oblique diameter ~12 No Aids fetal rotation
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Writer: Vandita Singh, Lucknow (GS India Nursing Group