Anatomy of normal pelvic and fetal skull

4th stage

نسائية
Lec-1
د.ولدان

23/11/2015

ANATOMY OF NORMAL PELVIS & FETAL SKULL
Knowlage of the anatomy of normal female pelvis, fetal skull & soft tissues is essential to understand mechanism of labour.
THE PELVIS
Normal female pelvis is the rounded gynaecoid pelvis occurs only in 40% of white women. There are three other types:
android
Anthropoid & plattypelloid pelvis.
THE PELVIC BRIM OR INLET
The pelvis is divided into true & false pelvis which are separated by pelvic brim.
The plane of pelvic brim is bounded in front by the symphysis pubis [the joint separating the two pubic bones] on each side by the upper margin of pubic bone, iliopectineal line & ala of sacrum. Posteriorly by promontary of the sacrum.
The anteroposterior diameter of brim [true conjugate] is 11cm & the transverse diameter is 13.5cm.
THE PELVIC MID CAVITY
Can be described as an area bounded in front by the middle of symphysis pubis ,on each side by pubic bone, obturator fascia & inner aspect of ischial bone & spines. Posteriorly bounded by the junction of second & third pieces of sacrum.
The cavity is roomy circular with anterioposterior & transverse diameters both measure 12cm.


THE PELVIC OUTLET
Is roughly diamond shaped & bounded infront by the lower margin of symphysis pubis, on each side by the descending ramus of pubic bone , ischial tuberosity & sacrotuberous ligament & posteriorly by the last piece of the sacrum.
In gynaecoid pelvis the subpubic arch is wide & tuberosities are far apart. The anterior-posterior diameter is 13.5cm & the transverse diameter is 11cm.

THE PELVIC FLOOR

It forms part of birth canal, it is formed by the two levator ani muscles which with their fascia form amusculofascial gutter during the second stage of labour with the opening of the vagina looking forward between sides of the gutter
THE PELVIC AXIS
Is an imaginary curve line shows the path which the centre of fetal head follows during its passage through the pelvis, it is obtained by taking several anteroposterior diameters of the pelvis & joining their centers.

PELVIC INCLINATION

Is the angle that any pelvic plane makes with the horizontal. In the erect position the brim is normally inclined at 60 degrees .
Pelvic outlet is inclined at about 25 degrees. On vaginal assessment sacral promontary cannot be reached with the examining finger in normal pelvis.
It is possible to estimate the diagonal conjugate vaginally which the distance between the promontory &lower margin of symphysis pubis is 12.5 cm.
The true conjugate between the promontory &upper margin of symphysis pubis is 11cm.

Android pelvis

It had many characteristics of male pelvis ,the brim is heart-shaped so the widest transverse diameter is much nearer to the sacrum, the side walls tend to converge, the ischial spines are prominent, the sacrum is straight & the subpubic arch is generally narrow with an angle of 70 or less.
Both the anteroposterior &transverse diameters of the outlet tend to be reduced. This type of pelvis is funnel-shaped with diameters decrease from above downwards so disproportion become worse as labour proceeds.
Anthropoid pelvis
The anteroposterior diameter of the brim exceeds the transverse diameter. It tends to be deep & the sacrum has six segments instead of five this is known as a high assimilation pelvis. Sacrum & axis of pelvic cavity are less curved than in gynecoid pelvis & subpubic may be little narrow, but the sacrosciatic notches are wide & anteroposterior diameter of the outlet is large .


Platypelloid pelvis
Is described as the simple [non-rachitic] flat pelvis. The brim is elliptical with a wide transverse diameter, the subpubic arch is wide & rounded.
Except in case of android pelvis, these variations have little effect on normal mechanism of labour unless there is considerable reduction in the size of pelvis.
Gynecoid pelvis
Anthropoid pelvis
Android pelvis
Platypelloid pelvis

THE FETAL SKULL

The bones ,sutures & fontanelles
Fetal skull is made of the vault, face & base. By the time of birth the bones of face & base are firmly united but the bones of the vault are not well ossified being joined by unossified membranes at the sutures.
The bones which form the vault are the parietal bones, parts of occipital , frontal & temporal bones
Three sutures are of obstetric importance:
SAGITTAL SUTURE lies between the superior borders of the parietal bones
FRONTAL SUTURE is a forward continuation of the sagittal suture, lies between the two parts of frontal bone
CORONAL SUTURE lies between the anterior borders of the parietal bones & the posterior borders of frontal bones.

FONTANELLES

Are the junctions of various sutures;
ANTERIOR FONTANELLE OR BREGMA:
Lies where the sagittal, frontal & coronal sutures meet, is diamond shaped is present at birth & takes about 20 months to close.
POSTERIOR FONTANELLE:
Lies at the posterior end of the sagittal suture between the two parietal bones & occipital bone. Is triangular in shape &it closed soon after birth.


The area of fetal skull bounded by the two parietal eminences & the anterior & posterior fontanelles is termed the vertex.
DIAMETERS OF FETAL SKULL
Is divided into vertical, longitudinal & transverse diameters. The fetal head is ovoid in shape, there are different longitudinal diameters that may present in labour depending on the attitude of fetal head.
The longitudinal diameter that present in a well flexed head [vertex presentation] is suboccipito-bregmatic diameter . Ii usually 9.5 cm from suboccipital region to the centre of the anterior fontanelle.
If the head is less well flexed the suboccipitofrontal diameter is involved, is taken from the suboccipital region to the prominence of the forehead & measures 10cm.
With further extention of the head the occipitofrontal diameter presents which is measured from the root of the nose to the posterior fontanelle &is 11.5.
The greatest longitudinal diameter that may present is the mento-vertical ,which is taken from chin to the furthest point of vertex &measures 13cm.This is known as a brow presentation&is too large to pass through normal pelvis.
Extention of the fetal head beyond this point results in a smaller diameter presenting. The submento-bregmatic diameter is measured from below the chin to the anterior fontanelle &measures 9.5 cm, this is clinically a face presentation.
TRANSVERSE DIAMETER is the biparietal diameter is measured from one parietal eminence to the other, is 9.5cm.

4th stage

نسائية
Lec-2
د.ولدان

23/11/2015

NORMAL LABOUR
Labour is the whole process whereby the products of conception are expelled from the mother being naturally about term.
The uterus is divided into upper & lower segment that are fully formed at the end of first stage of labour. With each contraction the upper part of uterus [upper segment] muscles become shorter & thicker
The [lower segment] consist of lower part of body of the uterus & cervix which become thinner, taken up with contractions & then cervix dilated.
THE STAGES OF LABOUR
Labour is divided into three stages:
FIRST STAGE, or stage of dilatation from onset of true labour until the cervix is fully dilated.
SECOND STAGE, or stage of expulsion of fetus, from full dilatation of the cervix to delivery of fetus or fetuses.
THIRD STAGE, from delivery of fetus until the placenta & membranes are delivered.
Symptoms & signs of the onset of labour
Painful uterine contractions
The show
Rupture of membranes
Shortening & dilatation of cervix


The contractions
The uterus contracts irregularly & painlessly throughout pregnancy [Braxton hicks contractions], labour is recognized by regular & painful contraction during which uterus is felt harden with increase in duration, intensity & frequency. At end of first stage it may come every two to three minuttes & last 45 seconds to one minute.
The show
This is mucous discharge from the cervix mixed with little blood as a result of taking up of the internal os & separation of membranes.
Rupture of the membranes
The membranes may rupture at any time during labour & usually occurs towards the end of first stage of labour. Early rupture of membranes is more likely to occur if the presenting part not engaged or there is malpresentation.
Shortening & dilatation of the cervix
When labour begins the contraction & retraction of upper segment stretches the lower segment & upper part of cervix so that the internal os is pulled open, the cervix is dilated from above downwards becoming shorter until no projection into vagina is felt.

FIRST STAGE OF LABOUR

The uterine contraction &dilatation of internal os cause separation of chorion from decidua closest to it thus a small back of membranes is formed into the internal os, the head then comes down separates liquor amnii which is above it from that in the back called respectively the hind & fore waters. When membranes ruptured the fetal axis pressure comes into play; the upper pole of fetus is pressed on by the fundus of the uterus.
While the lower pole is pressed down onto the lower segment & cervix.
Normal first stage should not exceed 12 hours in a primegravida & 8 hours in multipara.
The character of pain is the same as spasmodic dysmenorrhoea caused by ischaemia of uterine muscles from compression of blood vessels in the wall of the uterus.
THE SECOND STAGE OF LABOUR
The presenting part is pushed down onto the pelvic floor, pelvic floor resistance has to be overcome by uterine contractions, aided by the action of the voluntary muscles of the abdominal wall & diaphragm. As contraction comes on patient takes a deep breath then bears down with all the force of her abdominal muscles, during height of pain there may be expiratory groans.
With each contraction the presenting part is forced down to pelvic floor, during intervals between contraction at first slipped back, after this with contraction & expulsive effort the head slowly moves down in a forward direction, when the widest diameter of the head distends the vulva it is said to be crowned. Then the head passed through vulva followed by the body in next contraction.
The third stage of labour
As the cavity of the uterus becomes smaller after delivery the placenta is shorn off the spongy layer of decidua basalis, further uterine contractions expel the placenta from the upper segment into the lower segment & vaginal vault this is called :separation of placenta. The placenta is expelled from vagina followed by membranes & any retroplacental blood clot.

THE MECHANISM OF LABOUR

This is referred to series of changes in position & attitude which the fetus undergoes during its passage through the birth canal.
The following items are used to describe the position of fetus in relation to the uterus & maternal pelvis;
Lie
The relation of the long axis of the fetus to the uterus, this may be longitudinal, oblique or transverse.
Presentation
Is that part of the fetus in or over the pelvic brim in relation to the cervix. If the head occupies the lower segment the presentation is cephalic, if is flexed on the spine the vertex presents.
If the head is fully extended this caused face presentation,& if is partly extended cause brow prsentation.
If the breech occupies lower segment termed
Podalic presentation. If the fetus lies obliquely caused shoulder presentation.
Position
The relationship between selected part of the presenting part of the fetus to maternal pelvis [the denominator]. With vertex presentation the denominator is the occiput, with face presentation it is the chin [mentum] &with breech presentation it is the sacrum. For each prsentation 4 positions are described, in vertex presentation; left & right occipitoanterior position [LOA&ROA] & left & right occipitoposterior position [LOP&ROP].
Attitude
Refers to the relation of different parts of the fetus to one another. Normally the head ‘back &limbs of fetus are flexed, in some abnormal presentation head or limbs may be extended.
The mechanism of labour in vertex presentation;
-Engagement.
-Descent.
-Flexion.
-Internal rotation.
-Extention.
-Restitution.
-External rotation.
-Shoulder rotation.
-Delivery of fetal body.
Engagement
The head normally enters the pelvis in the transverse position, engagement is occurred when the widest part of the presenting part has passed through the inlet. It occur in vast majority of nulliparous women prior to labour, but not in multipara.
If more than two-fifths of the fetal head is palpable abdominally then the head is not said to be engaged.


Descent
During the first stage &first phase of the second stage of labour descent of the fetus is secondary to uterine action .
In the second phase of the second stage of labour descent of fetus is helped by voluntary use of abdominal musculature.
Flexion
The fetal head may not always be completely flexed when it enters the pelvis. As the head descends into the narrower midcavity, flexion should occur.
This is probably as a passive movement in part due to the surrounding structures.
Internal rotation
This occurs because with a well- flexed head the occiput is leading &meets the sloping gutter of the levator ani muscles, which by their shape direct it anteriorly; the occiput rotate forward from the LOA or LOT position to lie under subpubic arch, with the sagittal suture in the anteroposterior diameter of the pelvic outlet.
Extension
The well flexed head now extends with the occiput escaping from underneath the the symphysis pubis & starting to distend the vulva,this is known as crowning of the head.
The head extends further &the occiput underneath the symphysis pubisalmost acts as a fulcrum point as the bregma,face & chin appear in succession over the posterior vaginal opening & perineal body.
Restitution
When the head is delivering, the occiput is directly anterior. As soon as it escapes from the vulva, the head aligns itself with the shoulders, which have entered the pelvis in the oblique position. The slight rotetion of the occiput through one-eighth of a circle is called restitution.
External rotation
In order to be delivered,the shoulders have to rotate into direct anterior-posterior plane.
When this occurs,the occiput rotate through a further one-eighth of a circle to the transverse position.
Shoulder rotation
When restitution & external rotation have occurred ,the shoulders will be in the anterior-posterior position. The anterior shoulder is under the symphysis pubis &delivers first & the posterior shoulder delivers subsequently.
Delivery of fetal body
Normally the rest of fetal body is delivered easily occasionally aided by lateral movement.