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Causes and onset of labour

Jun 23, 2015




2. LABOUR Labour is a physiologic process during which theproducts of conception (i.e., thefetus, membranes, umbilical cord and placenta) areexpelled outside the uterus 3. The following criteria should bepresent to call it normal labour:Spontaneous onsetSpontaneous expulsion,of a single,mature foetus,presented by vertex,through the birth canal,within a reasonable time (not less than 3 hours or more than 18 hours),without complications to the mother, or the foetus. 4. Onset of labourI. Characterized by The showTrue labour painsDilatation and effacement of cervixFormation of bag of forewaters 5. The show Show (bloody show) - sign of the impending onset of active labor - extrusion of mucus plug of the cervical canal Discharge of small amount of blood-tingedmucus from vagina5/34 6. True labour pains Uterine Contractions Characteristic of Labor ; muscular contractions, those of uterine smooth muscle of labor are painful cause of pain (not known definitely) hypoxia of contracted myometrium compression of nerve ganglia in cervix & lower uterus by the tightly interlocking muscle bundles stretching of cervix during dilatation stretching of peritoneum overlying the fundus 7. Interval between contractions: 10 minutes at the onset of the first stage diminishes gradually 1 minute or less in the second stage Periods of relaxation between contractions- essential to welfare of the fetus- unremitting contraction of uterus compromisesuteroplacental blood flow, cause fetal hypoxia Duration of contraction in active phaseDuration 30-90 seconds (average 60 sec)Pressure 20-60 mmHg (average 40 mmHg 8. Dilatation and effacement of cervix Effective force of the 1st stage of labor is uterinecontraction As the result of the action of these forces, twofundamental changes take place in the alreadyripened cervix effacement & dilatation The cervix is said to be completely (fully) dilated : 10 cm 9. Cervical Effacement obliteration or taking up of the cervix shortening of the cervical canal (2cm merecircular orifice with almost paper thin edge) muscular fibers at about the level of theinternal os are pulled upward or taken upinto the lower uterine segment 10. Formation of bag of forewatersThe process of cervical effacement and dilatation causes the formation of the forebag of amniotic fluid which is the leading portion of amniotic sac and fluid located in front of the presenting part 11. Phase 1Phase 2 Phase 3 Phase 4 QuiescenceActivationStimulationInvolution Prelude toPreparationProcessesParturient parturitionfor laborof laborrecoveryContractileUterineUterine Uterine Unresponsiveness, preparedness contraction, involutioncervical softening for labor,cervical dilation, cervical repair, cervical fetal and placentabreast feeding ripening expulsion (threestages of labor)ConceptionInitiation of onset ofparturitionlaborDelivery ofconceptusFertilityrestoredTHE PHASES OF PARTURITION 12. Phase I :uterine quiescence and cervical softening In this phase the inherent propensity ofmyometrium to contract is held in abeyance anduterine muscle is rendered unresponsive tonatural stimuli. Although some myometrial contraction of lowintensity and brief duration that do not causecervical dilatation are noted during this phase. Near the end of pregnancy ,contractions of thistype become more common, especielly inmultiparous women reffered to as Braxton Hickscontractions or false labour 13. Cervical softeningCervical softening is characterzed by an increase in tissue compliance,yet the cervix remains firm and unyielding.Cervical softening results from increased vascularity,glandular hypertrophy and hyperplasia,and compositional or structural changes of the extracellular matrix. 14. Phase I : biochemical andphysiological changesProgesterone and oestrogen: administration of progesterone receptor antagonist will promote some or all key features of onset of labour like cervical ripening ,increased cervical distensibility and increased uterine sensitivity to uterotonins. 15. Steroid hormone regulation of myometrial cell to cell communication ; progeterone causes decreased expression of contraction associated proteins and is also known to inhibit expression of gap junctional proteins and thus increases uterine quiescence. 16. G-protein coupled receptors thatpromote myometrial relaxation Beta Adrenoreceptors: adrenergic receptors mediatesGs-stimulated increases in adenylyl cyclase,increased levels ofcAMP and myometrial cell relaxation LH and hCG receptors :these receptors during pregnancyare greater before than during labour and activatesadenylyl cyclase by way of plasma membrane receptor Gs-linked system. Relaxin ;relaxin family peptide receptor-1-mediatesactivation of adenylyl cyclase and thus may promoteuterine relaxation 17. CRH ;synthesized in placenta and hypothalamus ,found to have dual role during pregnancy and labour,During pregnancy it binds the receptor CRH- R1production of cAMP and subsequent inhibition of myometrial activityAt term CRH can activate the Gq protein pathway which favours myometrial contraction 18. Prostaglandins ;most commonly considered as uterotonins however they have diverse effects and some acts as smooth muscle relaxantsBoth PGE2 and PGI2 could potentially act to maintain uterine quiscence by increasing cAMP signaling yet PGE2 can promote uterine contractility through binding to EP1 and EP2 receptors 19. Thus either the generation of specific prostaglandins or relative expression of the various prostaglandins receptors may determine myometrial response to prostaglandinsAtrial and brain natriuretic peptide and cGMP:cGMP levels can be stimulated by either ANP and BNP and promotes smooth muscle relaxation .BNP is secreted by amnion in large amounts and ANP is expressed in placenta 20. Accelerated uterotonin degradation In addition to pregnancy induced compoundsthat stimulates myometrial cell refractorinessthere are striking increases in enzymes thatdegrade or inactivates endogenously produceduterotonins which includes: PGDH and prostaglandins Oxytocinase and oxytocin Diamine oxidase and histamine Angiotensinases and angiotensin II PAF acetylhydrolase and PAF 21. Phase II :uterine activation andcervical ripening Myometrial changes include marked increase inoxytocin receptors ,prostglandin receptors and increasein number and surface area of gap junction proteinssuch as connexin43 .together these leads to increaseduterine irritability and responsiveness to uterotonins. Another critical change in phase 2 is formation of loweruterine segment from the isthmus .with thisdevelopment lightening occurs .it is also likely thatlower segment myometrium is unique from that ofupper segment resulting in distinct role for each inlabour.there are studies that reports an expressiongradient of oxytocin receptors with higher expressionin fundal myometrium. 22. Cervical ripening in phase2The transition from softening to ripening begins weeks or days before onset of contractions.The cervix is made up of only 10 to 15 percent smooth muscle and remaining is connective tissue which includes type 1,3 and 4 collagen,glycosaminoglycans, proteoglycans and elastin 23. During cervical ripening collagen fibrils are disorganized and there is increased spacing between fibrils and the total amount and composition of proteoglycans and glycosaminoglycans within the matrix are altered 24. Phase II: physiological and biochemical processesProgesterone functional withdrawl:this can be mediated through several mechanismsChanges in the relative expression of of nuclear progesterone receptor isoforms,PR- A,PR-B and PR-cChanges in relative expression of membrane bound progesterone receptors 25. Posttranslational modifications of progesterone receptorAlterations in progesterone receptor activity through changes in in the expression of co- activators or co-repressors that directly influence receptor functionLocal inactivation of progesterone by steroid- metabolizing enzymes or synthesis of a natural antagonist. 26. Oxytocin receptorsThere is an increase in myometrial oxytocin receptors and there activation results in increased phospholipase C activity and subsequent increase in cytosolic calcium and uterine contractility 27. RelaxinCauses remodeling of extracellular matrix of uterus,cervix,vagina, breast and pubic symphysis as well as promoting cell proliferation and inhibiting apoptosis. 28. Fetal contribution to initiation of labourUterine stretch and parturition is required for induction of contraction associated proteins.Stretch increases expression of gap junction protein-connexin 43,as well as oxytocin receptors. 29. Fetal endocrine cascade At term the fetal adrenal glands weigh same asthose in the adults and similar in size The daily production of steroid by adrenal glandsnear term is 100 to 200mg/day higher than 30 to40mg/day seen in adult glands at rest Fetal cortisol levels increase during the last weeksof gestation during the same period levels ofDHEA-S also increases significantly leading toincrease in maternal oestrogens particularlyestriol. 30. Placental CRH productionA CRH hormone identical to maternal and fetal hypothalamic CRH is synthesized by placenta in relatively large amounts.One important difference is that,unlike hypothalamic CRH which is under glucocorticoid negative feedback , cortisol has been shown to stimulate placental CRH production. 31. This ability makes it possible to create a feed forward endocrine cascade that does not end untill separation of fetus from placenta at delivery.Resulting high levels of CRH may modulate myometrial cotractility via interaction with the CRH receptor isoform CRH-R1d this isoform is known to enhance myometrial contractile response 32. It has also been proposed that cortisol affects the myometrium indirectly by stimula