The shortest diameter of pelvic outlet is:
A. Oblique
B. Bispinous
C. Intertuberous
D. Anteroposterior
Correct Answer: B. Bispinous
Explanation
The shortest diameter of the pelvic outlet is the intertuberous (bituberous) diameter.
Explanation:
The pelvic outlet has three important diameters:
- Anteroposterior (straight) diameter
- From lower border of symphysis pubis to tip of coccyx
- About 11.5 cm (can increase during labor due to backward movement of coccyx)
- Intertuberous (bituberous) diameter ✅
- Between the inner borders of the two ischial tuberosities
- About 10.5 cm
- Shortest and fixed diameter of the pelvic outlet
- Posterior sagittal diameter
- From midpoint of intertuberous diameter to sacrum
✅ Correct answer:
Intertuberous (bituberous) diameter
Other Options Details
These options refer to important diameters of the pelvis, mainly used in obstetrics to assess the adequacy of the birth canal.
Below is a clear explanation of each option:
A. Oblique diameter
- It runs from one sacroiliac joint to the opposite iliopubic eminence.
- Present at the pelvic inlet.
- Length is about 12–12.5 cm.
- Important because the fetal head often enters the pelvis in an oblique diameter.
B. Bispinous diameter (Interspinous diameter)
- Distance between the two ischial spines.
- This is the narrowest diameter of the mid-pelvis.
- Normal length is about 10–10.5 cm.
- Very important clinically because engagement and descent of the fetal head depend on it.
C. Intertuberous diameter (Bituberous diameter)
- Distance between the two ischial tuberosities.
- Located at the pelvic outlet.
- Normal length is about 11 cm.
- Determines the width of the pelvic outlet and affects the final stage of labor.
D. Anteroposterior diameter
- Runs from the pubic symphysis to the sacrum.
- Present at pelvic inlet, mid-pelvis, and outlet (with different names at each level).
- At the inlet, it is called the obstetric conjugate (~10.5–11 cm).
- Essential for assessing pelvic adequacy for vaginal delivery.
Summary
- Oblique → pelvic inlet
- Bispinous → mid-pelvis (narrowest)
- Intertuberous → pelvic outlet
- Anteroposterior → front-to-back diameter at all pelvic levels
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Writer: Vandita Singh, Lucknow (GS India Nursing Group