Pelvis measuring is the most important method of pelvis examination.

Most internal dimensions of the pelvis are inaccessible for measurement, therefore usually its external dimensions are measured, by which the internal ones are evaluated.

The pelvis is measured with the help of the pelvimeter.

Usually there are measured four main dimensions of the pelvis: three transversal and one straight. Distantia spinarumis the distance between the anterosuperior axes of the ilia. This dimension makes 25 cm. Distantia cristarumis the distance between the most distant points of the iliac crests; it makes 28 cm on average. Distantia trochantericais the distance between the greater trochanters. This dimension makes 21 cm.

Conjugata externa(external conjugate) is the straight dimension of pelvis. The woman is put on her side; the leg lying below is bent in the hip and knee joints, the other leg is straightened. The end of the pelvimeter is set in the middle of the superior-external border of the symphysis, the other end is pressed to the supersacral fossa, which is situated between the process of the fifth lumbar vertebra and the beginning of the first sacral vertebra. The external conjugate makes 20 cm.

Dimensions of small pelvis are of great importance in obstetric-practice since the course and completion of delivery depend on them. But most dimensions of small pelvis can not be measured directly. The large pelvis is not of big importance for the birth of a child, but it is possible to judge about the form and size of small pelvis by its dimensions. The small pelvis cavity is the space between its walls, limited from above and from below by the area of brim and the area of pelvic outlet. It looks like a cylinder truncated from the front backwards in such a way that the anterior part (directed to the womb) is three times as low as the posterior one (directed to the sacral bone). There are differentiated four planes in the small pelvis cavity: the area of brim, the pelvic plane of greatest dimensions, the third parallel pelvic plane, and the area of pelvic outlet.

The planes of small pelvis and their dimensions:

a) the area of brim is limited from behind with the promontory of sacral bone, on the sides — with the terminal lines of hip bones, from the front — with the upper margin of pubic bone and symphysis. There are differentiated four dimensions.

The straight dimension — the distance from the promontory of sacral bone to the most protrudent point of the superointernal margin of symphysis, it is also called the true or obstetric conjugate (conjugata vera), makes 11 cm. There is also distinguished the anatomic conjugate (conjugata anatomic) — the distance from the promontory of sacral bone to the upper margin of symphysis, it is by 0.3 cm larger than the obstetric one.

The transversal dimension — the distance between the utmost points of the arcuate lines of ilia (linea innominata); it makes 13 cm.

The oblique dimension (left and right) — the distance from the left sacroiliac joint (articulatio sacroiliaca) to the right iliopubic eminence (eminentia iliopubica) and vice versa; it makes 12 cm.

b) the pelvic plane of greatest dimensions is limited from behind by the junction of the second and third sacral vertebrae, from the sides — by the middle of femoral fossae, from the front — by the mid dle of the internal surface of symphysis. In this plane two dimensions are differentiated — straight and transversal.

The straight dimension — from the projection of the junction of the second and third sacral vertebrae to the middle of the internal surface of symphysis; it makes 12.5 cm.

The transversal dimension — between the middles of femoral fossae; it makes 12.5 cm.

c) the third parallel pelvic plane is limited from the front by the inferior margin of symphysis, from behind — by the sacrococcygeal joint, from the sides — by the axes of ischial bones. There are differen tiated two dimensions of the third parallel pelvic plane — straight and transversal.

The straight dimension — from the sacrococcygeal joint to the middle of the inferior margin of pubic symphysis; it makes 11 cm.

The transversal dimension — between the internal surfaces of ischial bones axes; it makes 10.5 cm;

d) the area of pelvic outlet is limited from the front by the infe rior margin of symphysis, from behind — by the pelvic bone apex, from the sides — by the internal surfaces of ishial tuberosities. The dimensions of the area of pelvic outlet are straight and transversal.

The straight dimension — the distance from the middle of the inferior margin of symphysis to the pelvic bone apex; it makes 9.5 cm (during delivery, when the fetal head is being born, the pelvic bone reclines by 1.5 cm and the straight dimension increases to 11 cm).

The transversal dimension — the distance between the internal surfaces of ishial tuberosities; it makes 11 cm.

The dimensions of the pelvic outlet can be measured directly. For this purpose the pregnant is put on her back, the legs are bent in the hip and knee joints, moved sideways and pulled to the stomach. The measurement is conducted with a measuring tape or a special pelvimeter.

The straight dimension is measured between the mentioned above landmarks. To measure the transversal dimension-one should add 1.5 cm to the obtained distance between the internal surfaces of ishial tuberosities (9.5 cm), taking into account the soft tissues thickness.

The line, which goes in the middle of all the straight dimensions of the planes, is called the axis of pelvis (the plane of pelvic canal). The pubic angle makes 90—100°, the angle of pelvic inclination — 55—60°. The height of symphysis is measured during vaginal examination and makes 3.5—4 cm.

The most important dimension for pelvis evaluation is the true conjugate, which can not be measured directly. Therefore it is calculated from the dimensions, which are accessible to measurements — the external and diagonal conjugates.

To find the true conjugate 8 cm are subtracted from the value of the external conjugate if the circumference of the radiocarpal articulation < 14 cm; 9 cm — if the circumference of the radiocarpal articulation makes 14—16 cm; 10 cm — if the circumference of the radiocarpal articulation > 16 cm. For example: 20 cm - 9 cm = 11 cm.

The diagonal conjugate is the distance from the inferior margin of symphysis to the most protrudent point of the sacral bone promontory. The diagonal conjugate is measured by means of vaginal examination.

When introduced into the vagina, the index and long fingers move along the hollow of sacrum to the promontory of sacra, the tip of the long finger is fixed on the promontory apex, and the edge of palm rests against the inferior margin of symphysis. The place, where the doctor's hand touches the inferior margin of symphysis, is marked with a finger of the other hand. After the fingers are taken out of the vagina, the distance from the tip of the long finger to the marked point of the palm edge encounter with the inferior margin of symphysis is measured with a measuring tape or a pelvimeter.

The diagonal conjugate makes 13 cm on average. If it is impossible to reach the sacral bone promontory with a fingertip, the diagonal conjugate dimension is considered close to normal.

In order to find the true conjugate one has to subtract 1.5—2 cm from the diagonal conjugate depending on the circumference of the radiocarpal articulation: if the circumference makes 15 cm — 1.5 cm, if it makes 16 cm and more — 2 cm.

The main external pelvis dimensions and diagonal conjugate are measured in all pregnant and parturient women without exception.

If examination shows that the main dimensions are irregular and narrow pelvis is suspected, additional measurements are conducted.

The pelvic muscles

The perineum forms the pelvic floor, closing its outlet. In obstetrics the notion of perineum is narrower than in anatomy, where perineum is the space between the posterior commissure of pudendal lips and the anterior margin of anus.

The floor of small pelvis is formed by two diaphragms — pelvic and urogenital.

The pelvic floor muscles consist of three layers.

The superficial (external) layer is formed by such muscles: ischiocavernous (m. ischicavernosus) — begins from the ishial tuberosity and interweaves with the clitoris tissues; bulbocavernous (m. bul-bospongiosus) — begins from the tendinous center of the perineum and attaches to the vaginal walls; the external sphincter muscle of anus (m. sphincter ani externus) — begins in the region of the pelvic bone apex, envelops the anus, and interweaves into the tendinous centre of the perineum; the superficial transverse muscle of perineum (m. transversum perinei superficialis) — begins from the ishial tuberosity and ends in the tendinous centre of the perineum.

The middle layer of the pelvic floor muscles consists of the urogenital diaphragm, which is located between the symphysis pubis, pubic and ischial bones in the form of a triangle. It is formed by the sphincter muscle of urethra (m. sphincter urethrae internum) and the deep transverse muscle of perineum (m. transversus perinei profundus).

The internal layer of the pelvic floor muscles is named the pelvic diaphragm. This is the strong binate elevator muscle of anus (m. levator ani), which consists of muscle bundles: pubococcygeal (m. pubo-coccygeus) and iliococcygeal (m. iliococcygeus). The coccygeal muscle (m. coccygeus) is rudimentary and attaches to the lower vertebrae of sacral and pelvic bones.