Obstetric Pelvic Diameters and Planes
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Direct answer
Obstetric pelvic diameters are the measured distances across the bony birth canal, read at three planes: the inlet, the midcavity and the outlet. The conjugate that matters most for the fetal head is the obstetric conjugate, about 10 cm, the shortest anteroposterior distance from the sacral promontory to the posterior surface of the symphysis. It is estimated clinically by subtracting 1.5 to 2 cm from the diagonal conjugate, which is the distance palpated on vaginal examination.
What you must remember
- True conjugate: sacral promontory to the upper border of the symphysis pubis, about 11 cm; a geometric measure that cannot be felt directly.
- Obstetric conjugate: sacral promontory to the nearest point on the posterior surface of the symphysis, about 10 cm; the real limit for the fetal head at the inlet.
- Diagonal conjugate: sacral promontory to the lower border of the symphysis, about 12.5 cm; the clinically palpable distance that overestimates the obstetric conjugate.
- Transverse inlet: maximum transverse diameter of the brim, about 13 cm, lying nearer the sacral end of the brim than the anterior end.
- Midcavity: the plane of the ischial spines, with an interspinous (bispinous) diameter of about 10 cm, usually the narrowest level of the true pelvis.
- Outlet: anteroposterior diameter from the sacrococcygeal joint to the lower symphysis, about 11.5 cm, and a transverse outlet of roughly 10 to 11 cm between the ischial tuberosities.
- Station: the ischial spines are the zero point, so a head at the spines is station zero and the outlet lies at plus five.
A worked pathway for a borderline inlet
Take a multigravida at 38 weeks with a cephalic presentation whose diagonal conjugate is palpated at 12 cm. The temptation is to read this as adequate, but the working number is the obstetric conjugate.
- Estimate the obstetric conjugate: Subtracting 1.5 to 2 cm from the diagonal conjugate gives an estimate of 10 to 10.5 cm, which sits at the lower edge of normal.
- Compare with the head: Set the estimate against the expected engaging diameter of the head. The comparison is reassuring when the head is well flexed, because the presenting diameter is then the suboccipito-bregmatic 9.5 cm.
- Check descent at the midcavity: The interspinous diameter of about 10 cm decides whether the head rotates and passes.
- Interpret a stalled head: If the head fails to descend through the midcavity despite adequate contractions, the problem is in the midplane.
- Decide on labour: A trial of labour is only justified when the head is clearly moving.
Measurement at the inlet is a clinical estimate rather than a precise value, so a single palpated distance is not proof of disproportion. Experienced clinicians combine the conjugate with the head's station, the moulding and the pattern of progress. Where the estimate is close to the threshold, a trial of labour under monitoring is the usual decision, with operative delivery reserved for failure to progress.
Where the exam tries to catch you
The same few errors recur in pelvic questions.
- Trap: the diagonal conjugate is the obstetric conjugate Correct: the diagonal conjugate is longer by roughly 1.5 to 2 cm, so using it overstates the space available.
- Trap: the true conjugate and the obstetric conjugate are the same measurement Correct: the true conjugate is the geometric distance, whereas the obstetric conjugate is the one that limits the fetal head.
- Trap: the midcavity and the outlet are the same plane Correct: the midcavity at the ischial spines is often the narrowest true pelvic plane, while the outlet lies at station plus five, and the station or the position of the presenting part shows which is meant.
A head fixed at the spines points to a midplane problem rather than an inlet problem.
Frequently asked questions
What is the normal obstetric conjugate?
The normal obstetric conjugate is about 10 cm, measured from the sacral promontory to the nearest point on the posterior surface of the symphysis pubis. It is the most important inlet diameter for fetal head passage in clinical practice.
How is the obstetric conjugate estimated on examination?
The obstetric conjugate is estimated by subtracting 1.5 to 2 cm from the diagonal conjugate, which is the distance from the sacral promontory to the lower border of the symphysis. This estimate is an approximation rather than a direct measurement.
Which pelvic plane is usually the narrowest?
The midcavity, at the level of the ischial spines, is commonly described as the narrowest plane of the true pelvis. Its interspinous diameter of about 10 cm is often the site where descent of the fetal head stops.
What is the transverse diameter of the pelvic inlet?
The transverse diameter of the pelvic inlet is about 13 cm at its widest point. This distance is larger than the anteroposterior inlet diameters, so the head usually enters the brim in the anteroposterior direction.