Pocket ObGyn – Clinical Pelvimetry
See Abbreviations
Pelvic Anatomy
- Pelvis: Sacrum, coccyx, & innomin Innomin = ilium, ischium, & pubis ® join sacrum at sacroiliac jnts & each other at symphysis pubis.
- Linea terminalis (aka innomin line): Divides false & true pelves
False pelvis: Above linea terminalis, bounded by lumbar vertebra, iliac fossa, & anter abdominal wall
True pelvis: Clinically important for parturition; it includes:
Post: Anter surface of the sacrum
Lateral: Inner surface of ischial bones
Anter: Pubic bones & ascending rami of ischial bones
Planes and Diameters of the Pelvis
- Obstetric Conjugate (OC; aka AP diameter): Obstetrically relevant Shortest distance btw the promontory of the sacrum & the symphysis pubis. Measured indirectly by subtracting 1.5–2 cm from the diagonal conjugate.
- Diagonal conjugate: Distance btw lower margin of symphysis to sacral Measured clinically w/ examining hand & used to calculate OC.
- Transverse diameter: Distance btw linea terminalis on either At right angle to obstetrical conjugate. Largest diameter of pelvis.
- Interspinous diameter: In midpelvis. Smallest pelvic diameter, but usually >10
Figure 9.1 Pelvic shapes
|
Gynecoid Anthropoid
Android Platypelloid |
(From Klossner NJ, Hatfield NT. Introductory Maternity & Pediatric Nursing. 2nd ed. Philadelphia, PA: Wolters Kluwer Health; 2010)
| Normal AP and transverse diameters of pelvis by shape | ||||
| Gynecoid | Anthropoid | Android | Platypelloid | |
| AP diameter
Transverse diameter |
12 cm
11 cm |
>12 cm
<12 cm |
12 cm
11 cm |
12 cm
10 cm |
| Description | “Ideal” | Upright oval | Heart shaped | Sideways oval |
Pelvic Shapes
- Caldwell & Moloy classification: Describes 4 ideal types, recognizing there are variations in pelvic Characterized primarily by the transverse & interspinous diameter.
- Gynecoid: Deemed “ideal” w/ wide pelvic inlet & outlet & straight sidewalls
- Anthropoid: Narrow transverse diameter but wide AP diameter
- Platypelloid: Wide inlet & outlet w/ narrow AP diameter & sacral inclination
- Android: Straight sidewalls w/ narrow subpubic arch & narrow incline of sacrum
Pelvimetry in OB Practice
- Clinical: Clinical exam of pelvis to predict CPD Clinical pelvimetry = poor predictor of CPD
- Radiologic pelvimetry: X-ray or MRI to predict Radiographic pelvimetry studies ® no impact on mat or neonat morbidity or mortality (Cochrane Database Syst Rev 2000:CD000161).
- Pelvimetry largely replaced by trial of No evid to recommend Cesarean deliv for concerns for CPD based on clinical or radiographic pelvimetry.