anatomy of maternal pelvis and fetal skull

Anatomy of female pelvis and fetal head
Pelvic anatomy
Bony pelvis
The bony pelvis is made of 4 bones: the sacrum, coccyx, and 2 innominate bones which are (composed of the ilium, ischium, and pubis). These are held together by the SIJ, SP, and the SCJ joints.

The anterior superior edge of the 1st sacral vertebra is called the promontory which is an important land mark in obstetrics

The true pelvis is important in obstetrics
the false pelvis is not important
Pelvic shapes (types):
We have 4 types or shapes of the bony pelvis and these are: the gynecoid, android, anthropoid, and finally the platypelloid.

1- The gynecoid: it is the classic female pelvis and is seen in about 50% of all the women

2- android pelvis: which is the typical male pelvis and found in < than 30% of women

3- anthropoid pelvis: is found in 20% of women

4- platypelloid pelvis: which is a flattened gynecoid pelvis and seen in 3% of women

Pelvic diameters:
1- the obstetric conjugate of the pelvic inlet: 11 cm
2- the bispinous diameter: 10.5 cm in the midcavity.
3- the bituberous diameter 11 cm in the pelvic outlet
4- the curve and length of the sacrum
5- and finally the subpubic angle


Clinical pelvimetry:
The diameters that can be assessed clinically are: the obstetric conjugate of the inlet by clinical assessment of the diagonal conjugate when the tip of the middle finger can not meet the promontory of the sacrum (while the 2 fingers are passed in the vagina and the index finger meets the pubis) then we subtract 1.5-2 cm will corresponds the obstetric conjugate (which is said to be adequate inlet).

Radiological assessment of the pelvis:
Is indicated when there is history of pelvic trauma or there is suspicion of pelvic abnormalities, and the MRI is more preferable than XR.


Dimensions of the fetal skull:
The fetal head is the largest and the least compressible part of the fetus
The fetal skull consists of a base and a vault (cranium) which consists of the occipital, parietal, frontal and temporal bones; these are easily compressible and interconnected by membranes and these features allow molding to occur

Diameters:
1-suboccipitobregmatic (9.5) cm this is the presenting anteroposterior diameter when the head is well flexed.
2-Occipitofrontal (11-12) cm: when the head is deflexed. Extend from the external protuberance of the occipital bone to the glabella.
3-Mentovertical (13.5) cm when the head is extended in brow presentation. It extends from the vertex to the chin.
4-Submentobregmatic (9.5) cm when hyper extended head in the face presentation.it extends from below the chin to the bregma