Female Pelvis

Female pelvis

Anatomical position of pelvis Tilted forwards and downwards ASIS and pubic tubercle should be in the same coronal plane. Hence the front of pubic symphysis faces downwards

The shape and size of the bony pelvis are important factors determining the outcome of labor. The pelvic inlet is the entrance to the true pelvis. The plane of the pelvic inlet is almost circular in a normal female (or gynaecoid pelvis) with a slightly greater transverse than anterior-posterior diameter.

Pelvic inlet an oblique plane joining the sacral promontory the arcuate and pectineal lines and the upper margin of the symphysis pubis is called the pelvic brim(pelvic inlet,linea terminalis) This divides the pelvis into greater(false) and lesser(true) pelvis

(major and minor pelvis)

Pelvic brim

Measurements of inlet (pelvimetry)

A-P diameter- about 11cm Transverse diameter- about 13..5cm (across the middles of the pelvic brim) Oblique diameter-12.5 cm (From iliopubic eminence to opposite sacroiliac joint)

Pelvic cavity The A-P diameter called the obstetric conjugate diameter is measured between the midpoint of the sacral promontory and the nearest point on the posterior surface of the pubic symphysis.

Pelvic outlet  

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The pelvic outlet is narrower than the inlet The A-P diameter is measured from the lower margin of pubic symphysis to the tip of the coccyx (10cm) The transverse diameter is between the two ischial tuberosities(10cm) The interspinous diameter is the shortest diameter in the pelvis(9cm) It is not necessary to remember the diameters of the pelvis in millimeters (or whatever unit one may choose!).

outlet

Pelvimetry in X-ray

Types of pelvis The Caldwell-Moloy (1933) classification Gynaecoid Android(male-like) Anthropoid(ape-like) Platypelloid(flat)

•Gynaecoid: Ideal shape, with round to slightly oval (obstetrical inlet slightly less transverse) inlet best chances for normal vaginal delivery.

Note the shapes of the inlet

The typical female pelvis has  

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broader greater sciatic notch broader ala of sacrum compared to the body of sacrum broader pubic bone compared to the acetabulum wider sub pubic angle

These four differences are easiest to remember! These four points illustrate how the

female pelvis is wider, to serve as the birth canal. 1.The broad greater sciatic notch increases the anteroposterior dimension. 2.The broad ala of sacrum increases the size of the inlet. 3.The broader pubic bone does the same to the inlet and the middle of the cavity. 4.The wide sub pubic angle pushes the ischial tuberosities farther apart, increasing the transverse diameter of the outlet. Also note that in the female pelvis the sacrum is thrust more backwards, increasing the A-P diameter of the outlet. There are many other 'differences', but these few are significant for us

M

F

Which is the female pelvis? Give four reasons for it

The true (T), obstetrical (O), and diagonal (D) conjugate diameters are indicated.