Mechanism of normal labor

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  • 1. Mechanism of normal labor Dr ERMIAS 1
  • 2. LABOUR • Physiologic process by which a fetus is expelled from the uterus to the outside world. • An increase in myometrial activity • precisely, a switch in the myometrial contractility from ”contractures” (long lasting, low frequency activity) to “contractions”(frequent, high intensity, high frequency activity) resulting in effacement and dilatation of uterine cervix. 2
  • 3. • Mean duration of human singleton pregnancy is 280days(40 weeks) from the last menstrual period. • “term pregnancy” refers to period from 37.0weeks to 42.0 weeks of gestation • “pre_term pregnancy”: onset of labour prior to 36 completed weeks of gestation • “post_term pregnancy”: pregnancy continuing beyond 42.0 weeks of gestation 3
  • 4. Diagnosis clinical diagnosis classic diagnosis of labor includes • Regular painful uterine contractions • Progressive cervical effacement and dilatation • Show + rupture of the fetal membranes 4
  • 5. Physiology of normal labour at term • factors responsible for initiation and maintenance of labor at term are not well defined. • Theories • progesterone withdrawal • oxytocin induction • fetal 5
  • 6. Phases of parturition Regulation of uterine activity during pregnancy is divided into four distinct physiologic phases. • Phase 0: state of functional uterine quiescence • Phase 1: phase of uterine activation (awakening) • Phase 2: phase of uterine stimulation • Phase 3: phase of uterine involution 6
  • 7. Mechanisms of labour • The ability of the fetus to successfully negotiate the pelvis during labour and delivery is dependent on the complex interactions of three variables –The power –The passenger –The passage 7
  • 8. The power • Force generated by uterine musculature • Characterized by frequency, amplitude(intensity),and duration. • Assessment may include, simple observation, manual palpation,external objective assessment techniques(external tocodynamometry), or direct measurement of intrauterine pressure using(internal manometry or pressure transducers) 8
  • 9. ……..CONT’D “adequate” uterine contractions • progressive cervical effacement and dilatation • 3_5 contractions in 10minutes or 7contractions in 15 minutes • 200_250montevidio units 9
  • 10. the passenger fetal variables that influence the course of labor and delivery • Absolute fetal size (macrosomia >4500gms) • Multifetal pregnancy • Lie: the longitudinal axis of the fetus in relation to the longitudinal axis of the uterus Either longitudinal, Transverse or oblique 10
  • 11. ….the passenger • presentation: the fetal part that directly overlies the pelvic inlet cephalic, breech, shoulder, funic, compound • Malpresentation is any presentation that is not cephalic with occiput leading, accounting for 5% of all term labor 11
  • 12. …passenger • attitude: position of head with regard to the fetal spines • position: the relationship of a nominated site of presenting part to the denominating location of the internal pelvis • Position can be most accurately assessed on transvaginal examination. 12
  • 13. Malposition is any position in labor which is not ROA,OA or LOA • presentations • Occiput • Breech • face • shoulder • Nominated sites »vertex »Sacrum »Mentum » Acromium 13
  • 14. N.B in both classifications the midpoint (0 station) the plane of the maternal ischial spines • station : measure of descent of the presenting part of the fetus through the birth canal. • old classification - by subjective arbitrary assignment of seven stations(-3 to +3) • new classification attempts to quantitate in centimeter the distance of the leading bony edge from the ischial spines of the maternal pelvis(-5 to +5). 14
  • 15. The passage consists of • the bony pelvis (composed of the sacrum, ilium,ischium and pubis) and • the resistance by the soft tissues of the pelvis. The pelvis has a series of planes broadly classified into • pelvic inlet, • Mid-pelvis cavity and • outlet. • The shape of the female bony pelvis can be classified as: gynecoid, anthropoid, android and platypeloid 15
  • 16. Stages of labor • three stages, First stage: the interval between the onset of labor and full cervical dilatation has been subdivided into three phases according to the rate of cervical dilatation • latent phase : the interval between the onset of labor and a point at which a change in the slope of the rate of cervical dilatation in noted usually 2-3cm. 16
  • 17. …First stage of labor • active phase : associated with a greater rate of cervical dilatation and usually begins at 3-4cm interval between 3-10cm of cervical dilatation • descent phase : coincides with the second stage of labor 17
  • 18. • second stage : interval between full cervical dialitation(10cm) and delivery of an infant. Characterized by descent of the presenting part through the maternal pelvis culminating in expulsion of the fetus. • Third stage :the delivery of the placenta and fetal membranes usually lasts less than 30 minutes. 18
  • 19. Cardinal movements in labor • the changes on position of fetal head during its passage through the birth canal. • Because of the asymmetry of the shape of both the fetal head and the maternal bony pelvis, such rotations are required for the fetus to successfully negotiate the birth canal. 19
  • 20. seven discrete movements of the fetus 1. engagement : passage of the widest diameter of the presenting part to a level below the plane of the pelvic inlet. The sagital suture is frequently deflected either posteriorly to the promontory or anteriorly to the symphysis pubis such lateral deflection the fetal head is known as anterior and posterior asynclitism. 2. Descent : downward passage of the presenting part through the pelvis. 20
  • 21. 21 Descent: As the fetal head engages and descends, it assumes an occiput transverse position because that is the widest pelvic diameter available for the widest part of the fetal head.
  • 22. …cardinal movements 3. Flexion : flexion of the fetal head occurs passively as the head descends due to the shape of the bony pelvis and the resistance offered by the soft tissues.. 4. Internal rotation : rotation of the presenting part from its original position (usually transverse with regard to the birth canal) to the anterior position as it passes through the pelvis. 22
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  • 25. …cardinal movements 5.Extension : occurs once the fetus has reached the level of the interoitus, bringing the base of occiput in contact to the inferior margin at the symphysis pubis. 6.External rotation (restitution) : is return of the fetal head to the correct anatomic position in relation to the fetal torso. 7.Expulsion : delivery of the rest of the fetal body. 25
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  • 28. • Cardinal movements are better observed by partogram 28