Norm of Pelvimetry (Digital Pelvic Examination)
Pelvic inlet: 11.0 cm.
Pelvic outlet: 8.0 cm.
Usage of Pelvimetry (Digital Pelvic Examination)
Evaluation of the pelvic adequacy for vaginal delivery. Clinical, noninvasive estimations of the important pelvic measurements in pregnancy. If there have been pelvic injuries, known bony abnormalities, or a previous difficult labor, a radiograph or a CT scan is needed to fully assess the adequacy for vaginal delivery.
Description of Pelvimetry (Digital Pelvic Examination)
Digital pelvimetry may be performed by the physician or nurse midwife during pregnancy or nearing delivery to estimate the adequacy of the woman's pelvic measurements for vaginal delivery. Any abnormality noted with this method is confirmed by other methods, and pelvicephalography is used to fully assess the prospects of normal delivery. The utility of this procedure has been questioned in the literature, as retrospective reviews indicate that a trial of labor is permitted, regardless of the pelvimetry findings (Wong et al, 2003).
Professional Considerations of Pelvimetry (Digital Pelvic Examination)
Consent form not required.
- Obtain rubber gloves, lubricant, and a ruler or Thom's pelvimeters.
- Position the woman on the examination table in the dorsal lithotomy position with her feet supported in stirrups.
- 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.
- Pelvic inlet measurement: The examiner inserts these fingers into the vagina, using the middle finger to locate the lower border of the symphysis pubis and the sacral promontory. To measure the diagonal conjugate, the other hand is used to indicate where the pubis makes contact with the proximal part of the hand. This distance is calculated in centimeters. The obstetric conjugate is calculated by subtraction of 1.5 cm from the length of the diagonal conjugate. The true conjugate is calculated by subtraction of 1.0 cm from the diagonal conjugate. Because the obstetric conjugate is the narrowest anteroposterior diameter through which the fetus must travel, radiographic examination for accurate measurement is helpful. To measure the obstetric conjugate on radiographic film, locate the inner point of the symphysis and measure back to the sacral promontory. This distance should be approximately 11.0 cm.
- Pelvic capacity measurement: The sagittal posterior diameter is the only midpelvis diameter that can be palpated. The fingers are inserted into the vagina, locating the sacrospinous ligament. This ligament is traced by palpation from the ischial spines to the sacrum. The sacrospinous ligament is usually three fingerbreadths long, or 4–5 cm. The capacity of the midpelvis, particularly the midplane, will give the examiner an idea of how labor will progress, if at all.
- Pelvic outlet measurement: The pelvic outlet is the area in which the fetal head crowns and extends for delivery. The pelvic outlet measurements can be obtained by palpation. The most important diameter is the obstetric anteroposterior outlet diameter. The flexibility and mobility of this diameter are usually assessed by palpation of the coccyx. With the finger inserted into the rectum while the thumb externally grasps the coccyx, the examiner attempts to move the coccyx downward. An immobile coccyx indicates a decreased outlet diameter. The anteroposterior outlet diameter is obtained by insertion of two fingers into the vagina and locating the tip of the sacrum and externally locating the symphysis pubis with the other hand. The distance between the fingers is marked. The suprapubic angle is estimated by placement of the thumbs, side by side, at the symphysis border. The fingers are then separated from the thumbs and placed flat against the thighs. The angle at which the fingers are able to separate from the thumbs is the suprapubic angle. If the suprapubic angle is narrow, minimal finger-thumb separation will occur. Another way to measure the suprapubic angle is to insert one finger into the vagina, locating the internal margin, while the other hand externally palpates the top of the symphysis pubis. An imaginary line is drawn between the two points and assessments of the depth and bend of the symphysis pubis are made. From these calculations, an estimate of the angle is made. Preferably the symphysis pubis is short and continues inward, allowing for an adequate obstetric conjugate. If it were bony and elongated, the fetus might have difficulty extending the head during delivery. The transverse outlet diameter is measured with the fist on Thom's pelvimetry position between the ischial tuberosities. Pelvimetry outlet measurements are important in the assessment of the potential for fetal head injuries and perineal tearing during the final stages of labor.
- Assist the woman to a position of comfort.
Client and Family Teaching
- The procedure takes 15 minutes.
- Explain the implications of the findings in relation to the type of delivery anticipated.
Factors That Affect Results
- The accuracy of the results depends on the skill and performance technique of the examiner.
- Outlet dystocia is a narrowing of the pubic arch and may make it difficult for the fetus to extend its head, resulting in the need for a forceps delivery.