The female pelvis and fetal skull
د.اسراء حميد المعيني 4th year2016-2017
1-The boundaries of female pelvis and types of female pelvis
2-Clinical types of female pelvis
3-The bony structures of fetal skull and its diameters
The bony pelvis is made of four bones :
The sacrum ,coccyx ,and two innominates(ilium, ischium and pubis).
These bones are held together by symphsis pubis ,sacroiliac joints and sacrococcygeal joint.
The sacrum consists of 5 fused vertebrae,the anterior –superior edge of the first vertebra is called sacral promontory,which protrudes slightly into the cavity of the pelvis.
he anterior surface of the sacrum is usually concave.it articulates with the illium at its upper segment ,with coccyx at its lower segment ,and with the sacrospinous and sacrotuberous ligaments laterally.
The coccyx is composed of three to five rudimentary vertebrae. it articulate with the sacrum.
The pelvic brim and inlet
The pelvic brim is the inlet of the pelvis and bounded in front by the symphysis pubis (the joint separating the two pubic bones) on each side by the upper margin of the pubic bone the ileopectineal line and the ala of the sacrum posteriorly by the promontory of the sacrum.
the normal transverse diameter in this plane is 13.5 cm and is wider than the anterior-posterior diameter which is normally 11cm ,angle of the inlet is normally 60 degree to the horizontal in the erect position.
The pelvic midcavity
The pelvic midcavity can be described as an area bounded in front by the middle of the symphysis pubis on each side by the pubic bone the obturator fascia and the inner aspect of the ischial bone and spine post by the junction of the2nd and 3rd section of the sacrum.
The cavity is almost rounded ,as the transverse and anterior diameter are similar at 12cm ,the ischial spine are palpable vaginally and are used as land mark to asses the descent of the head during vaginal examination (station) they are also used as land marks for providing an anesthesia block to the pudendal nerve.
Pudendal nerve passes behind and below the ischial spine on each
The pelvic out let
The pelvic outlet is bounded in front by the lower margin of the symphysis pubis on each side by the descending ramus of the pubic bone,the ischial tuberosity and the sacrotuberous ligament posteriorly by the last piece of sacrum, the AP diameter of the pelvic out let is 13.5 cm and the transverse diameter is 11cm .
Avariety of pelvic shapes has been described and these may contributed to difficulties in labor:
Present in 40%of women pelvic inlet is rounded
with transverse diameter larger than antero-posterior diameter
side wall is straight ,well rounded sacroscaitic notch,
well curved sacrum ,spacious sub pubic angle =90 degree, average prominence of spine ,head forced to occipital anterior position
20% of female ,long narrow oval inlet ,long antero-posterior diameter large posterior inclination of sacrum ,spine not prominent but close ,narrow subpubic angle ,precipitate occipital-posterior position and delivery in such
In 30% of women tringular inlet with flat post segment widest diameter closed to sacrum , side is convergent ,long and narrow sacrosciatic notch, shallow sacral curve ,narrow subpubic arch ,prominent spine ,forced to be occipit-trasverse position (funnel shape) deep trasverse arrest
Flattened gynaecoid pelvis 3%of female pelvis
oval shape inlet ,straight or divergent round sacrosciatic notch posterior inclination of sacrum wide bispinous diameter wide subpubic angle , fetal head engage in transverse diameter increased risk of obstructed labour.
The pelvic floor
This is formed by the two levator ani muscles which with their fascia form a musculofascial gutter during the 2nd stage of labour.
The perineal body is a codensation of fibrous and muscular tissue lying between the vagina and the anus .
It receives attachments of the posterior ends of the bulbocavernous muscles ,the medial ends of the superficial and deep transverse perineal muscles and the anterior fibers of the external anal sphincter ,it is always involved in a 2nd degree perineal tear and an episiotomy.
The Fetal Skull
The fetal skull is made up of the vault ,face ,base.
The sutures are the lines formed where the individual bony plates of the skull meets one another.
At the time of labour ,the sutures joining the bones of the face and the skull are firmly united the vault of the skull is formed by the parietal bones and parts of the occipital ,frontal and temporal bones.
Between these bones there are four membaranous sutures,the sagital ,frontal coronal and lumbdoidal sutures
The anterior fontanelle or bregma closed at 18 months (diamond shape)is at the junction of the sagittal ,frontal coronal sutures.
The posterior fontanelle triangular in shape lies at the junction of the sagittal and lambdoidal sutures between the two parietal bones and the occipital bone closed at 6-8 weeks of life.
It allow these bone to move together and even to overlap the parietal bones usually tend to slide over the frontal and occipital bones.
The bones themselves are compressible together these characteristics of the fetal skull allow a process called moulding to occur ,which effectively reduces the diameter of the fetal skull and encourages progress through the bony pelvis with out harming the under lying brain.
Vertex the area of the fetal skull bounded by the two parietal eminences and the anterior and posterior fontanelle.
Engament occurs when the widest diameter of the fetal presenting part has passed through the pelvic inlet
In vertex –biparietal
of the presenting part in the pelvis canal is define as its level above or below the plane of the ischial spines
= level zero,eacn 1 cm above or below given -1 and +1
when the biparietal diameter is parallel to the pelvic plane and the sagital suture is mid way between the anterior and posterior planes of the pelvis when this relationship not present the head is considered to be asynclitic.
Attitude of the fetal head refers to the degree of flexion and extension at the upper cervical spine.
The diameter of fetal skull.
Different longitudinal diameters are presented to the pelvis in labour depending on the attitude of the fetal head.
Well flexed head the longitudinal diameter ,is the suboccipito –bregmatic diameter 9.5 cm and measured from the sub occipital to the anterior fontanelle
longitudinal diameter that present in a less well flexed head such as is found in the occipito- posterior position is the Sub-occipito-frontal diameter and is measured from the suboccipital region to the prominence of the forehead 10 cm .
Further extension of the head Occipito-frontal daimeter present this is measured from the root of the nose to the posterior fontanelle and is 11.5 cm.
The largest longitudinal daimeter that may present is the Mentvertical which is taken from the chin to the furthest point of the vertex and measure 13 cm known as Brow presentation and it is usually too large to pass through the normal pelvis.
Extension of the fetal head beyond this point result in a smaller daimeter,submentobregmatic daimeter is measured below the chin to the anterior fontanelle and is 9.5cm this is clinically a face presentation
Transverse diameter of fetal skull are:
Biparietal =9.5 cm represents the largest transverse diameter located between the two parietal bones
Bitemporal diameter = 8 cm represents the shortest transverse diameter located between the two temporal bones
The clinical evaluation is started by assessing the pelvis inlet pelvic inlet: can be assess clinically for its anteroposterior diameter .
The obstetric conjugate can be estimated from the diagonal conjugate .
Diagonal conjugate: is obtained on clinical examination
is approximated by measuring from the lower border of pubis to the sacral promontory using the tip of the second figure and the point where the base of the index figure meets the pubis(12cm)
The obstetric conjugate is then estimated by subtracting 1.5 to 2 cm, depending on the height and inclination of the pubis(11.5cm)
Often the middle figure of the examining hand cannot reach the sacral promontory, thus the obstetric conjugate is considered adequate .
If the diagonal conjugate is greater than or equal to 11.5 cm the anteroposterior diameter of the inlet is considered to be adequate.
The anterior surface of the sacrum is then palpated to assess its curvature. The usual shape is concave .
Aflat or convex shape may indicate anteroposterior constriction throughout the pelvis.
The midpelvis: cannot accurately be measured clinically in either the anteropoterior or transverse diameter .
A reasonable estimate of the size of the mid pelvis ,however ,can be obtained as follows.
the pelvis side walls can be assessed to determine whether they are convergent rather than having the normal ,almost parallel,configuration
The ischial spines are palpated carefully to assess their prrominance and several passes are made between the spines to approximate the bispinous diameter 10.5 cm .The lenghth of the sacrospinous ligment is assessed by placing one figure on the ischial spine and on the sacrum in the midline .
The average length is 3 fingure breadths.
If the sacrospinous notch that is located lateral to the ligament can accommodate two-and half figure tips,the posterior midpelvis is most likely of adequate dimensions. short ligament suggests a forward inclination of the sacrum and a narwed sacrospinous notch.
pelvic outlet is assessed.
This is done by first placing a fist between the ischial tuberosities.An 8.5cm distance is considered an adequate transverse diameter .
The posterior sagittal measurement should also be greater than 8cm.
The infrapubic angle is assessed by placing thumb next to each inferior pubic ramus and then estimating the angle at which they meet.An angle of less than 90 degree is associated with a contracted transverse diameter in the midplane and out let.
Radiological assessment of the pelvis:
When an accurate measurement of the pelvis is indicated nuclear magnetic resonance may be used .The advantage of MRI over the X-Ray or CT for the pelvic assessment is the lack of ionizing radiation exposure.
1-Clinical evidence or obstetric history suggestive of pelvic abnormalities.
2-A history of pelvic trauma.