MECHANISM OF LABOUR Lateefa Al Dakhyel FRCSC, FACOG Assistant professor & consultant Obstetric & gynecology department Collage of medicine King Saud University

Lie, presentation, attitude, &position FETAL LIE The relation of the long axis of the fetus to that of the mother 1-Longitudinal lie -99% of labors at term 2-transverse lie  multiparty, placenta previa, hydramnious, & uterine anomalies 3-oblique lie -maternal & fetal axes cross @ 45 angle -most unstable & become longitudinal Or transverse at labor.

ATTITUDE or posture. In later months posture of the fetus folded on itself to accommodate the shape of the uterus (flexed head, thighs, knees &feet ,the arms crossed over the chest) Change from this flexed attitude can cause abnormal presentations

.'" ' I ! 1\ 11 A B ii F c (A) vertex (B) sinciput (C) brow D (D) face

Vertex presentation  occiput Face presentation  mentum Fetal position The relation of a chosen point of the fetal presenting part to the Rt or Lt side of the maternal birth canal The chosen point Vertex presentation  occiput Face presentation  mentum Breech presentation Sacrum Each presentation has two positions Rt or Lt Each position has 3 varieties : Ant, transverse, post OA ROA LOA ROT LOT ROP LOP OP

Lie, presentation, attitude, &position CEPHALIC PRESENTATION Head is flexed sharply  vertex / occiput presentation Head is extended sharply  face presentation Partially flexed  bregma presenting (sinciput presentation) Partially extended  brow presentation BREECH PRESENTATION Frank breech Complete breech Footling breech (incomplete breech) .

FREQUENCY OF VARIOUS PRESENTATIONS & POSITIONS AT TERM Vertex  96% 2/3 Lt 1/3 Rt Breech  3.5% Face 0.3% Shoulder 0.4%

MECHANISM OF LABOUR WITH OCCIPUT PRESENTATIONS THE CARDINAL MOVEMENTS OF LABOUR 1-ENGAGEMENT The greatest transverse diameter BPD passes through the pelvic inlet It may occur in the last few weeks of pregnancy or only in labour especially in multipara The fetus enters the pelvis in transverse or oblique diameter LOT  40% ROT 20% OP 20% ROP >LOP ROA / LOA 20%

THE CARDINAL MOVEMENTS OF LABOUR 2-DESCENT In nullipara engagement takes place before the onset of labour & further descent may not occur till the 2nd stage In multipara descent begins with engagement It is gradually progressive till the fetus is delivered It is affected by the uterine contractions & thinning of the lower segment

3-flexion The descending head meets resistance of pelvic floor, Cx & walls of the pelvis   flexion The shorter suboccipito-begmatic is substituted for the longer occipito-frontal

Lever action producing ftexion of the head; conversion from occipitofrontal to suboccipitobregmatic diameter typically reduces the anteroposterior diameter from nearly 12- to 9.5 cm.

4-INTERNAL ROTATION Turning of the head from the OT position  anteriorly towards the symphysis pubis ie. Occiput moves from transverse to ant 45º Less commonly OT  posteriorly towards the sacrum 135º It is not accomplished till the head has reached the spines The levator ani muscles form a V shaped sling that tend to rotate the vertex anteriorly

5-EXTENSION When the flexed head reaches the vulva it undergoes extension  the base of the occiput will be in direct contact with the inferior margin of the symphysis pubis Crowning  the largest diameter of the fetal head is encircled by the vulvar ring The head is born by further extension as the occiput, bregma (ant.fontanelle), forehead, nose, mouth & chin pass successively over the perineum

6-EXTERNAL ROTATION RESTITUTION After delivery of the head it returns to the position it occupied at engagement , the natural position relative to the shoulders (oblique position)Restitution Then the fetal body will rotate to bring one shoulder anterior behind the symphysis pubis ( biacromial diameter into the APD of the pelvic outlet) Restitution is followed by complete external rotation to transverse position (occiput lies to next to Lt maternal thigh) The ant shoulder slips under the pubis By lateral flexion of the fetal body the post shoulder will be delivered & the rest of the body will follow

2.Engagement;descent, flexion 6. Restitution (external rotation) Cardinal movements in the mechanism of labor and delivery, left occiput anterior position. 302 2.Engagement;descent, flexion 6. Restitution (external rotation) 3. Further descent, internal rotation 4. Complete rotation, beginning extension

OCCIPUT POSTERIOR POSITION Mechanism of labour is identical to OT & anterior varieties Usually more longer. The occiput rotate to the symphysis pubis through 135º instead of 90º or 45º If rotation does not occur direct occiput post (5-10%)or Partial rotation transverse arrest

,. ,0 Mechanism of labor for right occiput posterior position, anterior rotation.

Q.1-Commonest presentation during labor is 1-left occiput anterior position (LOA) 2-right occiput anterior (ROA) 3-occiput transverse (OT) 4-left occiput posterior (LOP) Q.2- about breach presentation which is correct 1-beech presentation is the more common in primeparous 2-incomlete breach can’t be delivered vaginally 3-breach presentation can’t be diagnosed clinically 4-all breach presentation should be delivered by C/S Q.3-during labor 1-engagment occur when BPD reach the pelvic outlet 2-engagment always occur before onset of labor in prime. 3-if internal rotation to SP is 135 degree it will be OP 4-if the head is flexed it will be face presentation

Thank you