Pelvimetry – Methods, Measurements & Obstetric Use
Pelvimetry is the measurement of the dimensions of the bony pelvis. It is especially important in obstetrics to assess whether the birth canal is wide enough for a vaginal delivery.
Things worth knowing about "Pelvimetry"
Pelvimetry is the measurement of the dimensions of the bony pelvis. It is especially important in obstetrics to assess whether the birth canal is wide enough for a vaginal delivery.
What is Pelvimetry?
Pelvimetry is a diagnostic procedure used to measure the dimensions of the bony pelvis. It is primarily used in obstetrics to assess whether the pelvis of a pregnant woman is large enough to allow the baby to pass through the birth canal during a vaginal delivery. Pelvimetry is also applied in orthopaedics and trauma surgery, for example when evaluating pelvic malalignment or injuries.
Why is Pelvimetry Performed?
The most important application of pelvimetry is in prenatal care. The primary aim is to detect a so-called contracted pelvis (pelvis angusta) early, which can complicate or prevent a natural birth. Other indications include:
- Assessment of pelvic asymmetry or deformity
- Planning of delivery management (vaginal delivery vs. caesarean section)
- Follow-up after pelvic fractures or surgeries
- Investigation of chronic pelvic pain
Methods of Pelvimetry
External Pelvimetry
External pelvimetry involves measuring palpable bony landmarks on the outside of the body using a specialised instrument called a pelvimeter. The following measurements are typically taken:
- Distantia spinarum: Distance between the anterior superior iliac spines (normal value approx. 25–26 cm)
- Distantia cristarum: Distance between the highest points of the iliac crests (normal value approx. 28–29 cm)
- Distantia trochanterica: Distance between the greater trochanters of the femur (normal value approx. 31–32 cm)
- External conjugate (Baudelocque diameter): Distance between the fifth lumbar vertebra and the upper edge of the pubic symphysis (normal value approx. 20 cm)
Internal Pelvimetry
Internal pelvimetry is performed vaginally. The main measurement obtained is the diagonal conjugate, which is the distance between the sacral promontory and the lower edge of the pubic symphysis. From this value, the obstetric conjugate (true conjugate) can be calculated. A value below 10 cm is considered indicative of a contracted pelvis.
Imaging-Based Pelvimetry
Modern imaging techniques allow more precise assessment of the pelvis:
- X-ray pelvimetry: Formerly widely used, but largely abandoned due to radiation exposure during pregnancy.
- MRI pelvimetry: Magnetic resonance imaging (MRI) is now considered the gold standard for imaging-based pelvimetry in pregnancy. It is radiation-free and provides accurate three-dimensional measurements of all relevant pelvic planes.
- CT pelvimetry: Used outside of pregnancy, for example in cases of pelvic trauma or pre-operative planning.
Key Pelvic Measurements in Obstetrics
Three pelvic planes and their diameters are critical for delivery:
- Pelvic inlet: The obstetric conjugate should be at least 10 cm.
- Mid-pelvis: The transverse diameter (interspinous distance between the ischial spines) should be at least 10 cm.
- Pelvic outlet: The distance between the ischial tuberosities should be at least 8 cm.
Relevance for Birth Planning
If pelvimetry indicates a contracted pelvis, this finding is incorporated into the delivery plan. Depending on the results, a caesarean section may be recommended. In many cases, a trial of labour is still attempted under close monitoring. Pelvimetry alone is not an absolute decision-making parameter – additional factors such as the position and estimated size of the baby, as well as uterine contractions, are also considered.
References
- Cunningham, F.G. et al.: Williams Obstetrics. 25th edition. McGraw-Hill Education, New York, 2018.
- Kowalski, R. et al.: MRI pelvimetry -- a modern reappraisal. Journal of Maternal-Fetal and Neonatal Medicine, 2019.
- Maharaj, D.: Assessing cephalopelvic disproportion: back to basics. Obstetrical and Gynecological Survey, 65(6):387–395, 2010.
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