Pelvimetry

General rules and transverse dimensions

To perform external pelvimetry, the physician stands to the right of the pregnant woman, facing her. The patient lies on her back with her legs extended and kept together. Measurements are taken using a pelvimeter: the arms of the instrument are held so that the thumbs and index fingers grasp the tips, and the measuring scale faces upward. The tips are pressed firmly against the bony landmarks, slightly compressing the subcutaneous adipose tissue.

Three main transverse parameters are distinguished:

• Distantia spinarum (25–26 cm) — the distance between the anterior superior iliac spines. The tips are fixed on the outer edges of the spines.
• Distantia cristarum (28–29 cm) — the distance between the most distant points of the iliac crests. To find this distance, the tips are moved upward from the spines along the outer edge of the crests.
• Distantia trochanterica (31–32 cm) — the distance between the most prominent parts of the greater trochanters of the femurs.

Clinical significance: normally, each successive transverse dimension exceeds the previous one by 3 cm. If this difference is smaller, the physician may suspect anatomical abnormalities in the pelvic structure.

Anteroposterior diameter and external conjugate

To indirectly estimate the anteroposterior diameter of the lesser pelvis, the external conjugate (Conjugata externa) is measured. Its normal length is 20–21 cm.

For this measurement, the patient changes position: she lies on her left side, flexes her left leg at the knee and hip joints, and keeps her right leg straight.

Pelvimeter fixation points:

1. Anteriorly — the midpoint of the upper outer edge of the pubic symphysis.
2. Posteriorly — the suprasacral fossa. It is located below the spinous process of the 5th lumbar vertebra and corresponds to the upper angle of the sacral rhomboid (Michaelis rhomboid).

How to calculate the true conjugate

The true conjugate is the anteroposterior diameter of the pelvic inlet. It cannot be measured directly from the outside, so calculation methods are used in obstetrics.

Calculation using the external conjugate 9 cm is subtracted from the Conjugata externa value. For instance, with an external conjugate of 20 cm, the true conjugate is 11 cm. However, this method has a margin of error: the subtracted value (Solovyov index) may vary depending on the thickness of the sacrum, symphysis, and the volume of the woman's soft tissues.

Calculation using the diagonal conjugate (Conjugata diagonalis) This is a more accurate method. The diagonal conjugate is the distance from the lower edge of the symphysis to the most prominent part of the sacral promontory. It is measured only during a vaginal examination.

• If the sacral promontory cannot be reached by the physician's middle finger, it means the distance exceeds 12.5–13 cm. Consequently, the anteroposterior diameter of the pelvic inlet is normal (≥11 cm).
• If the promontory is reached, the physician marks the point of contact with the lower edge of the symphysis on their hand. After withdrawing the hand, this distance is measured with a tape measure. To obtain the true conjugate, 1.5–2 cm is subtracted from the result.

Dimensions of the pelvic outlet plane

These parameters are assessed when pelvic outlet contraction is suspected. The patient lies on her back with her legs flexed at the hip and knee joints, abducted, and pulled up as close to the abdomen as possible.

Anteroposterior diameter of the pelvic outlet Measured with a standard pelvimeter from the midpoint of the lower edge of the symphysis to the tip of the coccyx. The scale usually reads about 11 cm. To find the true value, subtract 1.5 cm from the obtained figure (correction for soft tissue thickness). The normal true anteroposterior diameter of the outlet is 9.5 cm.

Transverse diameter of the pelvic outlet This is the distance between the inner surfaces of the ischial bones. It is technically difficult to measure. A tape measure or a special pelvimeter with intersecting arms is used. When calculating, a correction for the thickness of the subcutaneous adipose tissue in this area is necessarily added to the obtained value.

Class and exam questions

What else to study on this topic

• Solovyov index: how to assess pelvic bone thickness using wrist circumference
• Michaelis rhomboid: anatomical boundaries and diagnostic significance of asymmetry
• Detailed technique of vaginal examination to assess the diagonal conjugate
• Features of using a pelvimeter with intersecting arms

Other topics in the "Obstetrics" section

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