Pelvimetry

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Author: Mikael Häggström [notes 1]

Planning

Indication

A review in 2003 considered routine performance of pelvimetry to be a waste of time, a potential liability, and an unnecessary discomfort.[1] A woman's pelvis loosens up before birth by hormones, so an investigation before fetal passage does not represent the physiologic capacity of the birth canal. Still, pelvimetry can be indicated when a woman has had failure to progress in a previous pregnancy.

Choice of modality

Low-dose 3D CT can be used for estimating pelvimetry parameters.[2]

Measurement

Parameter Maximum intensity projections[2] Thin slices End points Normal measures
Pelvic inlet Transverse diameter of the pelvic inlet Low-dose CT of transverse diameter of pelvic inlet.jpg Low-dose CT scan of transverse diameter of the pelvic inlet, coronal plane, annotated.jpg The iliopectineal lines, at widest transverse distance. 13 to 14.5 cm.[2]
Obstetric conjugate Low-dose CT of obstetric conjugate.jpg
Median plane, 20 mm thick
Same, but may require minor side-to-side scrolling to visualize both end points. The line between the closest bony points of the sacral promontory and the pubic bone next to the symphysis 10 to 12 cm.[2]
Interspinous distance Low-dose CT of interspinous diameter.jpg Interspinous distance, thin slice, annotated.jpg
Axial plane
The line between the closest bone points of the ischial spines 9.5 to 11.5 cm.[3]
Pelvic outlet Sagittal pelvic outlet diameter Low-dose CT of sagittal pelvic outlet diameter.jpg Same, but may require minor side-to-side scrolling to visualize both end points. The closest bony points of the sacrococcygeal joint and the pubic bone next to the symphysis.[notes 2] 9.5 to 11.5 cm.[3]
Intertuberous diameter Low-dose CT scan of intertuberous diameter.jpg Low-dose CT scan of intertuberous diameter, axial plane, annotated.jpg
Axial plane
The closest bony points of the ischial tuberosities 10 to 12 cm.[3]

Interpretation

This only needs to be done if requested by the clinician, otherwise it is only necessary to state the numerical values of the parameters, in order to avoid implying any actual physiologic capacity of the pelvis. Particularly, an implication of a narrow passage increases the risk of choosing Caesarean section.

Normal Borderline Narrow
Pelvimetry interpretation[4]
Sum of inlet measures >24 cm 23- 24 cm < 23 cm
Sum of outlet measures >31,5 cm 29,5 – 31,5 cm <29,5 cm
Obstetric conjugate >11,0 9,5 – 11,0 <9,5

A sagittal outlet is < 8 cm or interspinous diameter is <7 cm is regarded as narrow, even if the sum of outlet measures is normal.[4]

Report

The length of each of the distances above, but make an interpretation of any narrowing only if requested, to decrease the risk of the patient receiving a knife wound to the abdomen.

If the images are taken during a pregnancy, note the presenting part of the fetus if projected.

Notes

  1. For a full list of contributors, see article history. Creators of images are attributed at the image description pages, seen by clicking on the images. See Radlines:Authorship for details.
  2. This is also called the obstetric anteroposterior diameter of the pelvic outlet, to distinguish from the anatomic one which includes the coccyx.a However, the coccyx is normally pushed away during childbirth by laxity in the sacrococcygeal joint.b
    a. Page 94 in: Neville F. Hacker, Joseph C. Gambone, Calvin J. Hobel (2009). Hacker & Moore's Essentials of Obstetrics and Gynecology (5 ed.). Elsevier Health Sciences. ISBN 9781437725162. 
    b. Page 239 in: Wayne R. Cohen, Emanuel A. Friedman (2011).
    Labor and Delivery Care: A Practical Guide . John Wiley & Sons. ISBN 9781119971542. 

References

  1. "A retrospective review of performance and utility of routine clinical pelvimetry ". Family Medicine 36 (7): 505–7. 2004. PMID 15243832. 
  2. 2.0 2.1 2.2 2.3 "Pelvimetry by Three-Dimensional Computed Tomography in Non-Pregnant Multiparous Women Who Delivered Vaginally ". Polish Journal of Radiology 81: 219–27. 2016. doi:10.12659/PJR.896380. PMID 27231494. 
  3. 3.0 3.1 3.2 "Magnetic resonance pelvimetry for trial of labour after a previous caesarean section ". Sultan Qaboos University Medical Journal 10 (2): 210–4. August 2010. PMID 21509231. 
  4. 4.0 4.1 Kira Kersting (2017-01-04). Kronoberg County Council.