Normal Labor PDF

13 Normal Labor CHAPTER OUTLINE „ Causes of Onset of Labor „ Anatomy of Labor „ Management of First Stage of ▶ Contractile System of the „ Clinical Course of First Stage of Labor...

13 Normal Labor CHAPTER OUTLINE „ Causes of Onset of Labor „ Anatomy of Labor „ Management of First Stage of ▶ Contractile System of the „ Clinical Course of First Stage of Labor Myometrium Labor „ Management of Second Stage of „ Physiology of Normal Labor „ Clinical Course of Second Stage of Labor „ Events in First Stage of Labor Labor ▶ Immediate Care of the Newborn „ Events in Second Stage of Labor „ Clinical Course of Third Stage of „ Management of Third Stage of „ Events in Third Stage of Labor Labor Labor „ Mechanism of Normal Labor ▶ Place of Delivery ▶ Active Management of Third „ Management of Normal Labor Stage of Labor (AMTSL) LABOR Based on the formula, labor starts approximately on the expected date in 4%, 1 week on either side in 50%, 2 DEFINITION: Series of events that take place in the weeks earlier and 1 week later in 80%, at 42 weeks in 10%, genital organs in an effort to expel the viable products and at 43 weeks plus in 4%. of conception (fetus, placenta and the membranes) out of the womb through the vagina into the outer world is called ‘labor’. It may occur prior to 37 completed weeks, CAUSES OF ONSET OF LABOR when it is called the preterm labor. Labor is characterized The precise mechanism of initiation of human labor is by the presence of regular uterine contractions with still obscure. Endocrine, biochemical and mechanical effacement and dilatation of the cervix and fetal descent. A stretch pathways as obtained from animal experiments, however, put forth the following hypotheses. parturient is a patient in labor and parturition is the process of giving birth. Delivery is the expulsion or extraction „ Uterine distension: Stretching effect on the myometrium of a viable fetus out of the womb. It is not synonymous by the growing fetus and liquor amnii can explain the went with labor; delivery can take place without labor as in onset of labor at least in twins or polyhydramnios. elective cesarean section. Delivery may be vaginal, either Uterine stretch increases gap junction proteins, spontaneous or aided, or it may be abdominal. receptors for oxytocin and specific contraction- NORMAL LABOR (EUTOCIA): Labor is called normal if it associated proteins (CAPs). fulfils the following criteria: „ Fetoplacental contribution: Cascade of events 1. Spontaneous in onset and at term spontaners activate fetal hypothalamic-pituitary-adrenal axis 2. With vertex presentation vertexpresenter prior to onset of labor → increased CRH → increased 3. Without undue prolongation release of ACTH → fetal adrenals → increased cortisol 4. Natural termination with minimal aids mail.EEiia secretion → accelerated production of estrogen and prostaglandins from the placenta (Fig. 13.1). 5. Without having any complications affecting the health „ Estrogen: The probable mechanisms are: of the mother and/or the baby. — Increases the release of oxytocin from maternal ABNORMAL LABOR (DYSTOCIA): Any deviation from the pituitary. definition of normal labor is called abnormal labor. Thus, labor in a case with presentation other than vertex — Promotes the synthesis of myometrial receptors or having some complications even with vertex pres- for oxytocin (by 100–200 folds), prostaglandins and entation affecting the course of labor or modifying the increase in gap junctions in myometrial cells. nature of termination or adversely affecting the mater- — Accelerates lysosomal disintegration in the dec- nal and/or fetal prognosis is called abnormal labor. idual and amnion cells resulting in increased pro- staglandin (PGF2a) synthesis. DATE OF ONSET OF LABOR: It is very much unpredictable to foretell precisely the exact date of onset of labor. It is — Stimulates the synthesis of myometrial contractile not only varies from case to case but even in different protein—actomyosin through cAMP. pregnancies of the same individual. Calculation based — Increases the excitability of the myometrial cell on Naegele’s formula can only give a rough guide. membranes.  Chapter 13 Normal Labor 109 Fig. 13.1: Initiation of parturition „ Progesterone: Increased fetal production of dehy- decidual cells and myometrium. Synthesis is triggered droepiandrosterone sulfate (DHEA-S) and cortisol by—rise in estrogen level, glucocorticoids, mechanical inhibits the conversion of fetal pregnenolone to prog- stretching in late pregnancy, increase in cytokines esterone. Progesterone levels therefore fall before (IL–1, 6, TNF), infection, vaginal examination and labor. It is the alteration in the estrogen: progesterone separation or rupture of the membranes. Prostaglandins ratio rather than the fall in the absolute concentration enhance gap junction (intramembranous gap between of progesterone, which is linked with prostaglandin two cells through which stimulus flows) formation. synthesis. Biochemical Mechanisms Involved in the Synthesis of „ Prostaglandins: They are the important factors, Prostaglandins (Flowchart 13.1) which initiate and maintain labor. The major sites of Phospholipase A2 in the lysosomes of the fetal membranes synthesis of prostaglandins are—amnion, chorion, near term → esterified arachidonic acid → formation of free FLOWCHART 13.1: POSSIBLE MECHANISM IN INITIATION OF LABOR Abbreviations: CAPs, Contraction-associated proteins; NO, Nitric oxide; PGE2, Prostaglandin E2 110 Textbook of Obstetrics arachidonic acid → synthesis of prostaglandins through pros- t Intracellular Ca++ → calmodulin Ca++ → MLCK → phosphor- taglandin synthetase. Prostaglandins (E2 and F2a) diffuse in ylated myosin + actin → myometrial contraction. the myometrium → act directly at the sarcoplasmic reticulum t Decrease of intracellular Ca++ (or its shift to the storage sites) → inhibit intracellular cAMP generation → increase local free → dephosphorylation of myosin light chain → inactivation calcium ions → uterine contraction. Once the arachidonic acid of myosin light chain kinase → myometrial relaxation. cascade is initiated, prostaglandins themselves will activate lyso- somal enzyme systems. The prostaglandin synthesis reaches Uterine muscles have two types of adrenergic receptors— a peak during the birth of placenta probably contributing to its (1) a receptors, which on stimulation, produce a decrease in expulsion and to the control of postpartum hemorrhage. cyclic AMP (adenosine monophosphate) and result in contraction of the uterus and (2) β receptors, which on stimulation, „ Oxytocin and myometrial oxytocin receptors: produce rise in cyclic AMP and result in inhibition of uterine (i) Large number of oxytocin receptors are present contraction. in the fundus compared to the lower segment and the cervix. FALSE PAIN (Synonym: false labor, spurious labor): It is (ii) Receptor number increases during pregnancy found more in primigravidae than in parous women. reaching maximum during labor. It usually appears prior to the onset of true labor pain (iii) Receptor sensitivity increases during labor. by 1 or 2 weeks in primigravidae and by a few days in (iv) Oxytocin stimulate synthesis and release of PGs multiparae. Such pains are probably due to stretching of (E2 and F2a) from amnion and decidua. Vaginal the cervix and lower uterine segment with consequent examination and amniotomy (stretching of the irritation of the neighboring ganglia. lower genital tract), cause rise in maternal plasma oxytocin level (Ferguson reflex). Fetal plasma PRELABOR (Synonym: premonitory stage): The premoni- oxytocin level is found increased during sponta- tory stage may begin 2–3 weeks before the onset of true neous labor compared to that of mother. Its role labor in primigravidae and a few days before in mul- in human labor is not yet established. tiparae. The features are inconsistent and may consist „ Neurological factor: Although labor may start in of the following: denervated uterus, labor may be also initiated through „ Lightening: A few weeks prior to the onset of labor nerve pathways. Both a and β adrenergic receptors especially in primigravidae, the presenting part sinks are present in the myometrium; estrogen causing into the true pelvis. It is due to active pulling up of the the a receptors and progesterone the β receptors to lower pole of the uterus around the presenting part. It function predominantly. The contractile response is signifies incorporation of the lower uterine segment initiated through the a receptors of the postganglionic into the wall of the uterus. This diminishes the fundal nerve fibers in and around the cervix, and the lower height and hence minimizes the pressure on the dia- part of the uterus. This is based on observation that phragm (Figs 13.2A and B). The mother experiences onset of labor occurs following stripping or low rup- a sense of relief from the mechanical cardiorespiratory ture of the membranes. embarrassment. There may be frequency of micturition or constipation due to mechanical factor—pressure by CONTRACTILE SYSTEM OF THE MYOMETRIUM the engaged presenting part. It is a welcome sign as it The basic elements involved in the uterine contractile rules out cephalopelvic disproportion and other condi- systems are: (a) Actin, (b) myosin, (c) adenosine triphos- tions preventing the head from entering the pelvic inlet. phate (ATP), (d) the enzyme myosin light chain kinase „ Cervical changes: A few days prior to the onset of (MLCK) and (e) Ca++. labor, cervix becomes ripe. A ripe cervix is (a) soft, (b) Structural unit of a myometrial cell is myofibril which cont- 80% effaced (