Norm of Pelvimetry and Pelvicephalography
Pelvic Inlet (Anteroposterior Diameters)
|Diagonal conjugate||12.5 cm|
|Obstetric conjugate||11.0 cm|
|True conjugate||11.5 cm|
Pelvic Cavity (Midpelvis)
|Anteroposterior diameter||11.5–12.0 cm|
|Posterior sagittal diameter||4.5–5.0 cm|
|Transverse diameter||10.5 cm|
|Anteroposterior diameter||9.5 cm|
|Obstetric anteroposterior diameter||11.5 cm|
|Transverse diameter||8.0 cm|
|Suprapubic angle||85–90 degrees|
Usage of Pelvimetry and Pelvicephalography
Evaluation of pelvic adequacy for vaginal delivery when any of the following conditions are present: labor has been dysfunctional or slow; fetal positioning is breech, the fetal head fails to engage, or other abnormal positioning in labor occurs, especially in primigravidas, when maternal pelvic adequacy is questioned; history of pelvic fracture or injury or congenital deformity or disease, such as rickets, polio, or hip dysplasia, may affect the bony pelvis or hips. Examination of very small women or those with kyphoscoliosis or dwarfism. May be indicated when the physician is considering oxytocin administration. It is not the pelvic measurements alone but the pelvic measurements in relation to the size of the fetal head that are important to ensure safe delivery for these indicators. Pelvimetry measurements may be used when previous deliveries have been difficult or have produced large infants or in previous deliveries with an unplanned forceps delivery or non-elective cesarean section before another pregnancy. The radiographic tests are not performed as often as they were a decade ago. A trial of labor is usually permitted, regardless of the results of clinical pelvimetry.
Description of Pelvimetry and Pelvicephalography
Pelvimetry is measurement of the internal dimensions of the bony pelvis, usually to determine the adequacy and shape of the maternal pelvis in relationship to fetal size and positioning for or during vaginal delivery. Estimates of pelvic measurements may be performed digitally during the physical examination by an obstetrician, nurse midwife, or trained obstetrical nurse (see pelvic measurements under Pelvimetry, Diagnostic [Pelvic examination, Digital]). Other methods for more specific measurements of the pelvic outlet capacity and the fetal head size may be performed by radiography, CT scan, or MRI. Ultrasound pelvimetry is not considered accurate at this time. Pelvicephalography is a measurement of the fetal head diameters by a radiologic measurement. It is performed with special methods used to correct for radiographic distortion and magnification. Pelvimetry by radiography, ultrasonography, or computed tomography requires a physician's prescription.
During pregnancy, risks of cumulative radiation exposure to the fetus from this and other previous or future imaging studies must be weighed against the benefits of the procedure. Although formal limits for client exposure are relative to this risk:benefit comparison, the United States Nuclear Regulatory Commission requires that the cumulative dose equivalent to an embryo/ fetus from occupational exposure not exceed 0.5 rem (5 mSv). Radiation dosage to the fetus is proportional to the distance of the anatomy studied from the abdomen and decreases as pregnancy progresses. For pregnant clients, consult the radiologist/ radiology department to obtain estimated fetal radiation exposure from this procedure.
Professional Considerations of Pelvimetry and Pelvicephalography
Consent form NOT required.
- Prepare the client for transport to the appropriate radiology department.
- Radiographic pelvimetry is completed in the radiology department. The client is positioned carefully for an erect lateral view of the pelvis and a supine AP view of the pelvis. It is important that exact measurement of the woman's position and distance from the film at the time of radiography be taken for a correction factor used in the computation of pelvic measurements. The disadvantages include radiation hazards to the fetus and the fact that radiography alone is no longer considered reliable as a tool to diagnose problems with labor and delivery.
- Computed tomograph pelvimetry is considered more accurate and easier to perform. There is less radiation exposure to the fetus and less chance of distortion if the woman is positioned correctly on the table. Three views are taken: anterior, lateral, and axial. Electronic calipers are used to take the numerical pelvic measurements. CT is particularly useful in women with a history of pelvic fractures and before delivery for any situation except those in which a cesarean section is already planned.
- Ultrasonography is not yet clinically helpful. A radiograph is also required, and a fetal pelvic index that estimates proportion or disproportion for vaginal delivery must be computed.
- MRI is considered quite accurate and allows for imagery of the complete fetus as well as the mother's pelvis and pelvic measurements. The MRI has the advantage of evaluation of the soft tissues of the pelvic region as reasons for dystocia and uses NO radiation to the fetus. MRI is costly and difficult to access in emergency situations.
- Cephalopelvic proportion is use of the above radiographic techniques late in the pregnancy or during a difficult labor to assess the mother's pelvic dimensions as relates specifically to the size of the fetal head and position.
- Knowledge of the course of normal labor and delivery and full understanding of the correction factors for radiographic pelvimetry are essential.
- For digital examination, the lengths of the first two fingers on either hand of the examiner are measured in centimeters. These fingers should be used for obtaining all measurements.
- Assist the woman to a position of comfort.
- Assess the parents' readiness for new roles; include information on feeding, supplies, safety, and referral agencies.
Client and Family Teaching
- The procedure takes 15 minutes.
- The client must remove clothes and put on a gown.
- Explain the relationship of the results to the type of delivery—vaginal versus cesarean.
Factors That Affect Results
- Emotional support is more likely needed during this test than at other times during pregnancy.