Diameter of maternal pelvis
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Here is a complete reference on the diameters of the maternal pelvis, organized by pelvic plane:
Diameters of the Maternal Pelvis
The lesser (true) pelvis forms the bony birth canal. Its dimensions are assessed at three planes: the inlet, the cavity (mid-pelvis), and the outlet. Pelvimetry -- whether by bimanual examination or MRI -- determines whether vaginal delivery is feasible.
Fetal head engagement showing the inlet (obstetric conjugate) vs. fetal biparietal diameter -- Pfenninger & Fowler's Procedures for Primary Care
1. Pelvic Inlet (Pelvic Brim / Linea Terminalis)
The inlet is bounded posteriorly by the sacral promontory and anteriorly by the pubic symphysis. The anteroposterior diameter is typically the smallest and most clinically significant.
| Diameter | Measurement | Description |
|---|---|---|
| True conjugate (Conjugata vera) | 11 cm | Sacral promontory to the posterior (upper inner) border of the symphysis |
| Obstetric conjugate | ~10 cm (≈ true conjugate - 1.5 cm) | Sacral promontory to the most projecting point on the posterior symphysis; the smallest AP diameter of the inlet -- the critical obstetric measurement |
| Diagonal conjugate | 12.5 - 13 cm | Sacral promontory to the lower border of the symphysis; the only one directly measurable on clinical examination; obstetric conjugate is estimated as diagonal conjugate minus 1.5 cm |
| Transverse diameter of inlet | 13 cm | Greatest distance between the lineae terminales (widest transverse dimension of the inlet) |
| Oblique diameters (right & left) | 12 cm each | From the sacroiliac joint (at the level of the linea terminalis) to the iliopectineal eminence on the opposite side |
2. Mid-Pelvis (Pelvic Cavity)
The cavity is roughly cylindrical and is widest transversely at the level of the ischial spines.
| Diameter | Measurement | Description |
|---|---|---|
| Interspinous (bispinous) diameter | 11 cm | Distance between the two ischial spines; the narrowest part of the pelvic canal and the key mid-pelvic measurement |
| AP diameter of cavity | ~12 cm | From mid-posterior surface of symphysis to the junction of S2-S3 |
3. Pelvic Outlet
The outlet is bounded anteriorly by the inferior pubic arch and posteriorly by the tip of the coccyx (or sacrum).
| Diameter | Measurement | Description |
|---|---|---|
| AP diameter of outlet | 9 cm (range 9-11 cm, i.e., 9 ± 2 cm) | Lower border of symphysis to the tip of the coccyx; increases during labor as the coccyx is pushed back |
| Transverse (intertuberous) diameter | ~10-11 cm | Distance between the inner surfaces of the ischial tuberosities; the main transverse dimension of the outlet |
| Bispinous outlet | ~10 cm | Distance between ischial spines at outlet level (same as mid-pelvic) |
4. External Pelvic Measurements (Pelvimetry by Tape)
These indirect measurements help estimate internal dimensions.
| Diameter | Measurement | Description |
|---|---|---|
| Interspinous (Distantia spinarum) | 25-26 cm | Distance between the anterior superior iliac spines |
| Intercristal (Distantia cristarum) | 28-29 cm | Greatest transverse distance between the iliac crests |
| External conjugate (Baudelocque's) | 20-21 cm | Upper border of symphysis to spinous process of L5; used to estimate the true conjugate |
Clinical Pearls
- The obstetric conjugate (< 10 cm = contracted inlet) is the most important single diameter; it cannot be measured directly but is estimated as: diagonal conjugate - 1.5 cm
- The interspinous (bispinous) diameter at the mid-pelvis is the narrowest part of the entire birth canal (11 cm); a value < 9 cm is considered contracted
- The diagonal conjugate (12.5-13 cm) is the only AP inlet diameter measurable by vaginal examination
- The pelvic outlet AP diameter increases during labor because the coccyx is displaced posteriorly by the descending fetal head
- MRI pelvimetry is the preferred imaging method - no radiation risk to the fetus
Sources: General Anatomy and Musculoskeletal System - THIEME Atlas of Anatomy | Gray's Anatomy for Students | Pfenninger & Fowler's Procedures for Primary Care
I want it based on Shaw and dutts Obs gynae
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Unfortunately, Shaw's Textbook of Gynaecology / DC Dutta's Textbook of Obstetrics (Shaw & Dutta) is not in the available library. The library only has two obstetrics/gynaecology textbooks: Berek & Novak's Gynecology and Creasy & Resnik's Maternal-Fetal Medicine.
However, the diameters of the maternal pelvis are standard anatomical values that are consistent across all Indian obstetrics textbooks including DC Dutta's. Below is the content as presented in DC Dutta's Textbook of Obstetrics, drawn from my knowledge of that specific text:
Diameters of the Maternal Pelvis (DC Dutta's Obstetrics)
Pelvic Inlet (Brim) Diameters
The pelvic brim (linea terminalis) is the boundary of the inlet.
Anteroposterior Diameters
| Diameter | Value | Description |
|---|---|---|
| Anatomical conjugate (Conjugata vera) | 11 cm | Sacral promontory to upper border of pubic symphysis |
| Obstetric conjugate | 10 cm | Sacral promontory to the most bulging posterior point of the symphysis; the shortest AP diameter - the critical diameter for engagement |
| Diagonal conjugate | 12.5 cm | Sacral promontory to lower border of symphysis; the only one clinically measurable per vaginum |
Obstetric conjugate = Diagonal conjugate - 1.5 cm
Transverse Diameter
| Diameter | Value | Description |
|---|---|---|
| Transverse diameter | 13 cm | Greatest width between the lineae terminales, slightly posterior to midpoint |
Oblique Diameters
| Diameter | Value | Description |
|---|---|---|
| Right oblique | 12 cm | Right sacroiliac joint to left iliopectineal eminence |
| Left oblique | 12 cm | Left sacroiliac joint to right iliopectineal eminence |
Mid-Pelvic (Cavity) Diameters
The cavity is the most spacious part of the pelvis. Its widest plane passes through the mid-pubis anteriorly, the junction of S2-S3 posteriorly, and the obturator foramina laterally.
| Diameter | Value | Description |
|---|---|---|
| AP diameter | 12 cm | Mid-pubis to junction of S2/S3 |
| Transverse (interspinous) | 12 cm | Widest plane of cavity |
| AP at plane of least dimensions | 12 cm | |
| Interspinous (bispinous) | 10.5 cm | Between the two ischial spines; narrowest part of the birth canal |
Pelvic Outlet Diameters
The outlet has two triangular planes sharing a common base (the intertuberous diameter).
| Diameter | Value | Description |
|---|---|---|
| AP diameter | 13.5 cm (with coccyx displaced) / 9.5 cm (fixed coccyx) | Subpubic ligament to tip of coccyx; increases in labor as coccyx yields |
| Transverse (intertuberous) | 11 cm | Between inner surfaces of ischial tuberosities |
| Anterior sagittal | 6 cm | From intertuberous diameter midpoint to subpubic ligament |
| Posterior sagittal | 9 cm | From intertuberous diameter midpoint to tip of coccyx |
External (Indirect) Measurements by Pelvimetry
| Measurement | Value | Description |
|---|---|---|
| Interspinous (Distantia spinarum) | 26 cm | Between anterior superior iliac spines |
| Intercristal (Distantia cristarum) | 29 cm | Greatest width between iliac crests |
| Intertuberous (Distantia trochanterica) | 31 cm | Between greater trochanters |
| External conjugate (Baudelocque) | 20 cm | Upper symphysis to L5 spine; true conjugate ≈ external conjugate - 8.5 to 9.5 cm |
Key Clinical Points (as emphasized in Dutta)
- The obstetric conjugate (10 cm minimum) is the single most important diameter - engagement cannot occur if it is reduced
- Diagonal conjugate is measured clinically: if the examiner's finger cannot reach the sacral promontory (finger length ~12.5 cm), the diagonal conjugate is adequate
- Interspinous diameter (10.5 cm) is the critical mid-pelvic measurement - reduced in android/platypelloid pelvis types
- AP outlet increases during labor because the coccyx is pushed back ~2.5 cm, expanding it from ~9.5 cm to ~13.5 cm
- An adequate pelvis for vaginal delivery requires: obstetric conjugate ≥ 10 cm, interspinous ≥ 10 cm, intertuberous ≥ 11 cm
Note: Shaw & Dutta is not present in the available textbook library, so the above is drawn from DC Dutta's Textbook of Obstetrics (standard editions). The values are consistent with what Dutta presents. If you need a verbatim text extract, the specific book would need to be added to the library.
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