Maternal bony pelvis and fetal head

Maternal bony pelvis andfetal head

Objectives of this lectureIntroduction to normal labour and vaginal

delivery passages, passenger, and power.The student should know the types of female

pelvis.understand the importance of the dimensions of

the bony pelvis of the pregnant woman indetermining the progress of labour and the modeof delivery.

What are the methods for assessment of pelvicdimensions.

Know the dimensions of the fetal skull.Understand how the attitude of the fetal head

effect these dimensions.

• Labour can be defined as the process bywhich regular painful contractions bringabout effacement and dilatation of the cervixand descent of the presenting part, leadingto expulsion of the fetus and the placentafrom the mother.

• A doctor or midwife who manages labourmust be aware of the normal anatomy andphysiology of the mother and fetus, whatdistinguishes an abnormal from a normallabour, and when it is appropriate tointervene

Bony pelvis

• The bony pelvis is made of 4bones: the sacrum, coccyx, and2 innominate bones which are(composed of the ilium,ischium, and pubis). These areheld together by the SIJ, SP, andthe SCJ joints.

The bony pelvis.

(a) Inlet: Beanshaped.

(b) Mid-cavity:Circular.

(c)Outlet:Diamondshaped.

The pelvic brim or inlet

The pelvic axis

describesimaginarycurved line,a path thatthe centreof the fetalhead musttake duringits passagethrough thepelvis

The pelvic mid-cavityThe pelvic mid-cavity can be described as an area

bounded in front by the middle of the symphysispubis,

on each side by the pubic bone, the obturator fasciaand the inner aspect of the ischial bone and spines,

and posteriorly by the junction of the second and thirdsections of the sacrum.

The cavity is almost round, as the transverse andanterior diameters are similar at 12 cm.

The ischial spines are palpated vaginally and are used as

landmarks to assess the descent of the head on vaginalexamination (station). They are also used as landmarks forproviding an anaesthetic block to the pudendal nerve.

The bony pelvis.

(a) Inlet: Beanshaped.

(b) Mid-cavity:Circular.

(c)Outlet:Diamondshaped.

The pelvic outletThe pelvic outlet is boundedin front by the lower margin of thesymphysis pubis,on each side by the descending ramus ofthe pubic bone, the ischial tuberosity andthe sacrotuberous ligament,and posteriorly by the last piece of thesacrum.The AP diameter of the pelvic outlet is13.5 cm and the transverse diameter is 11cm

The bony pelvis.

(a) Inlet: Beanshaped.

(b) Mid-cavity:Circular.

(c)Outlet:Diamondshaped.

Pelvic diameters:

These represent the spaceavailable for the fetal headwhen it passes throughthe pelvis during labour

1. the obstetric conjugate of thepelvic inlet: 11 cm

2. the bispinous diameter: 10.5 cmin the midcavity.

3. the bituberous diameter 11 cm inthe pelvic outlet

4. the curve and length of thesacrum

5. and finally the subpubic angle

Pelvic shapes (types)We have 4 types or shapes of the

bony pelvis and these are: thegynecoid, 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 andcharacterized by the following:

• Rounded to bean shape inlet, side walls arestraight, ischial spines are of averageprominence, well curved sacrum, wide subpubicarch , Suitable for vaginal delivery

android pelvis: which is the typical male pelvisand found in < than 30% of women andcharacterized by:Heart shape inlet (triangular), convergent sidewall (funnel shape) with prominent spines,straight sacrum, and narrow subpubic arch.Associated with deep transverse arrest

anthropoid pelvis: is found in 20% ofwomen andAssociated with occipito- posteriorposition during labour

platypelloid pelvis: which is aflattened gynecoid pelvis and seenin 3% of women and is associatedwith obstructed labour

Clinical pelvimetry:The diameters that can be assessedclinically are: the obstetric conjugateof the inlet by clinical assessment ofthe diagonal conjugate when the tipof the middle finger can not meet thepromontory of the sacrum (while the2 fingers are passed in the vaginaand the index finger meets the pubis)then we subtract 1.5-2 cm willcorresponds the obstetric conjugate

Then we assess the curvatureof the sacrum by palpating itsanterior surface.Then the midpelvis is assessedbut it is difficult to do itclinically unless the pelvic sidewalls are apparently convergent

the bispinous dimension alsocan be assessed by palpatingthe prominence of the spines,in addition the width of thesacrosiatic notch should beassessed.

And the final step is theassessment of the outlet byplacing a fist between theischial tuberosities, adimension of 8.5 cm isadequate transverse diameter.And the subpubic arch of less

The perineumThe final obstacle to be negotiated by the

fetus during labour is the perineum. Theperineal body is a condensation of fibrousand muscular tissue lying between thevagina and the anus. It receivesattachments of the posterior ends of thebulbo-cavernous muscles, the medial endsof the superficial and deep transverseperineal muscles, and the anterior fibres ofthe external anal sphincter. It is alwaysinvolved in a second-degree perineal tearand an episiotomy.

Dimensions of the fetal skull

• The fetal head is the largest andthe least compressible part of thefetus

• The fetal skull consists of a baseand a vault (cranium) whichconsists of the occipital, parietal,frontal and temporal bones

these are easily compressibleand interconnected bymembranes and these featuresallow molding to occur whichmeans the overlap of thesebones under pressure andchanging their shape toconform to maternal pelvis

•You have to know many terms: *fontanelle [anterior (bregma) *and posterior (lambda)],*nasion,*glabella, *vertex, *and the occiput

suboccipitobregmatic (9.5) cm this isthe presenting anteroposteriordiameter when the head is wellflexed. It extends from theundersurface of the occipital boneto the center of the bregma.

Occipitofrontal (11) cm: when thehead is deflexed. Extend fromthe external protuberance ofthe occipitalbone to theglabella.

Mentovertical (13.5) cm whenthe head is extended inbrow presentation. Itextends from the vertex tothe chin.

Submentobregmatic (9.5) cmwhen hyper extended headin the face presentation.itextends from below the chinto the bregma.

moulding