5. Physiology of labour.pdf

Physiology of labour & The puerperium Physiology of Labour  The cervix, which is long, firm, and closed, with a protective mucus plug, softens, shortens, thins out (effacement) and dilates for labour to progress.  The uterus changes from a state of relaxation to an active state of regular, stro...

Physiology of labour & The puerperium Physiology of Labour  The cervix, which is long, firm, and closed, with a protective mucus plug, softens, shortens, thins out (effacement) and dilates for labour to progress.  The uterus changes from a state of relaxation to an active state of regular, strong, frequent contractions to facilitate transit of the fetus through the birth canal.  Each contraction is followed by a resting phase in order to maintain placental blood flow and adequate perfusion of the fetus.  The pressure of the presenting part on the pelvic floor muscles as the fetus descends from the midpelvis to the pelvic outlet produces a maternal urge to push, enhanced further by stretching of the perineum.  The onset of labour occurs when the factors that inhibit contractions are overtaken by the actions of factors that do the opposite.  Both mother and fetus appear to contribute to this process. The uterus  Myometrial cells of the uterus contain filaments of actin and myosin, which bring about contractions when intracellular calcium increases.  Prostaglandins and oxytocin increase intracellular free calcium ions, whereas beta-adrenergic compounds and calcium-channel blockers decrease it.  Progressive shortening ofthe uterine smooth muscle cells is called retraction and occurs in the the upper part of the uterus. This retraction process results in development of the thicker, actively contracting ‘upper segment’ and the lower segment of the uterus becomes thinner and more stretched.  As a result of this the cervix is ‘taken up’ (effacement) into the lower segment of the uterus.  This effaces and then dilates cervix, and the fetus descends in response to this directional force. The uterus Myocytes of the uterus contract in a coordinated way. There is cell-to cell communication by means of gap junctions, which facilitate electrical current between cells, appear in significant numbers at Term.  Prostaglandins stimulate the gap junction's formation, while beta-adrenergic compounds inhibit them. A Uterine pacemaker from which contractions originate probably exists but been demonstrated histologically. Contractions  Uterine contractions are involuntary in nature.  Throughout labour, they occur at intervals of 2–4 minutes and are described in terms of the frequency within a 10-minute period ( 2 in 10 increasing to 4–5 in 10 in advanced labour).  Their duration also varies during labour, from 30 to 60 seconds or occasionally longer.  The frequency of contractions can be recorded on a cardiotocograph (CTG) using a pressure transducer (tocodynamometer) positioned on the abdomen at the fundus of the uterus.  The intensity or amplitude of the intrauterine pressure generated with each contraction averages between 30 and 60 mmHg. The cervix  The cervix contains myocytes and fibroblasts separated by a ‘ground substance’ made up of extracellular matrix molecules.  The collagen, fibronectin and dermatan sulphate (a proteoglycan) keep the cervix firm and closed, during the early stages of pregnancy.  Under the influence of prostaglandins there is increase in proteolytic activity and a reduction in collagen and elastin. Interleukins bring about invasion by neutrophils.  Dermatan sulphate is replaced by hyaluronic acid, which results in an increase in water content of the cervix. This causes cervical softening or ‘ripening’, so that contractions, can bring about the processes of effacement and dilatation Hormonal factors  Progesterone maintains uterine relaxation by suppressing prostaglandin production, preventing oxytocin release. Estrogens opposes the action of progesterone.  Prior to labour, there is a reduction in progesterone receptors and an increase in the concentration of estrogen.  Prostaglandin synthesis by the chorion and the decidua is enhanced, leading to an increase in calcium influx into the myometrial cells.  The production of corticotrophin-releasing hormone (CRH) by the placenta increases in concentration towards term and potentiates the action of prostaglandins and oxytocin on myometrial contractility.  The fetal pituitary secretes oxytocin and fetal adrenal gland produces cortisol, which stimulates the conversion of progesterone to oestrogen. Hormonal factors As labour becomes established, the output of oxytocin increases through the 'Fergusson reflex’. Pressure from the fetal presenting part against the cervix is relayed via a reflex arc involving the spinal cord and results in increased oxytocin release from the maternal posterior pituitary. Diagnosis of labour  The onset of labor can be defined as the presence of strong regular painful contractions resulting in progressive cervical change. Therefore, a diagnosis of labor requires more than one vaginal examinations.  The diagnosis is suspected when a woman presents with contractionlike pains and is confirmed vaginal examination reveals effacement and dilatation of the cervix.  Loss of a ‘show’ (a blood-stained plug of mucus passed from the cervix) or spontaneous rupture of the membranes (SROM) does not define the onset of labor, although these events may occur around the same time. Stages of labour  Labour can be divided into three stages. The definitions of these stages rely predominantly on anatomical criteria, and in real terms the moment of transition from first to second stage may not be apparent.  The important events are the maternal urge to push, which usually corresponds with full dilatation of the cervix and the baby’s head resting on the perineum.  The average duration of a first labour is 8 hours, and that of a subsequent labour 5 hours. First labour rarely lasts more than 18 hours, and second and subsequent labors not usually more than 12 hours. First stage  This describes the time from the diagnosis of labour to full dilatation of the cervix (10 cm). The first stage of labour can be divided into two phases.  The ‘latent phase’ is the time between the onset of regular painful contractions and 3–4 cm cervical dilatation. During this time, the cervix becomes ‘fully effaced’.  Effacement is a process by which the cervix shortens in length as it becomes incorporated into the lower segment of the uterus.  It usually lasts between 3 and 8 hours, being shorter in multiparous women. Second stage  Dilatation is expressed in centimeters from 0 to 10 cm.  This describes the time from full dilatation of the cervix to delivery of the fetus.  The second stage of labour may also be subdivided into two phases. The 'passive phase’ describes the time between full dilatation and the onset of involuntary expulsive contractions. There is no maternal urge to push, and the fetal head is still relatively high in the pelvis.  The second phase is called the ‘active second stage’. There is a maternal urge to push because the fetal head is low(often visible), causing a reflex need to ‘bear down’. In a normal labour, the second stage is often diagnosed at this late point because the maternal urge to push prompts the midwife to perform a vaginal examination Second stage If a woman never reaches a point of involuntary pushing, the active second stage is said to begin when she starts making voluntary pushing efforts directed by her midwife. Conventionally, a normal active second stage should last no longer than 2 hours in a nulliparous woman and 1 hour in women who delivered vaginally before. A passive second stage of 1 or 2 hours is usually recommended to allow the head to rotate and descend prior to active pushing. Third stage This is the time from delivery of the fetus or fetuses until complete delivery of the placenta(e) and membranes. The placenta is usually delivered within a few minutes of the birth of the baby. A third stage lasting more than 30 minutes is defined as abnormal, unless the woman has opted for ‘physiological management 'in which case it is reasonable to extend this definition to 60 minutes. The duration of labour There is no ideal length of labour for all women, but morbidity increases when labour is too fast (precipitous) or two slow (prolonged). Prolonged labor is labor lasting longer than 12 hours in nulliparous women and 8 hours in multiparous women. Precipitous labour is defined as expulsion of the fetus within less than 3 hours of the onset of regular contractions. Puerperium Introduction  The puerperium refers to the 6-week period following completion of the third stage of labour, when considerable adjustments occur before return to the prepregnant state. For those with complex medical problems, the early puerperium is especially dangerous and most maternal deaths occur during this time.  During this period of physiological change, the mother is also vulnerable to psychological disturbances, which may be aggravated by adverse social circumstances.  Adequate understanding and support from her partner and family are crucial.  Difficulty in coping with the newborn infant occurs more frequently with the first baby, and vigilant surveillance is therefore necessary by the community midwife, general practitioner (GP) and health visitor. Uterine involution  Involution is the process by which the postpartum uterus, weighing about 1 kg, returns to its pre-pregnancy state of less than 100 g.  Immediately after delivery, the uterine fundus lies about 4 cm below the umbilicus or, more accurately, 12 cm above the symphysis pubis. However, by 2 weeks, the uterus becomes no longer palpable above the symphysis  Involution occurs by a process of autolysis, whereby muscle cells diminish in size as a result of enzymatic digestion of cytoplasm. The excess protein produced from autolysis is absorbed into the bloodstream and excreted in the urine. Involution appears to be accelerated by the release of oxytocin in women who are breastfeeding, as the uterus is smaller than in those who are bottle feeding.  A delay in involution in the absence of any other signs or symptoms (e.g., bleeding) is of no clinical significance. Involution of the uterus. (A) Day 1, 18-week sized uterus (just below the umbilicus); (B) day 7, 14-week sized uterus; (C) day 14, 12-week sized uterus. Uterus is larger following caesarean section and in multiparous women. Signs of delayed involution Artefact. Full bladder. Loaded rectum. Retained products of conception (or clots). Uterine infection. Fibroids. Broad ligament haematoma. Genital tract changes  Following delivery of the placenta, the lower segment of the uterus and the cervix appear flabby and there may be small cervical lacerations.  In the first few days the cervix can readily admit two fingers; by the end of the first week, it should become increasingly difficult to pass more than one finger, and certainly by the end of the second week the internal os should be closed.  However, the external os can remain open permanently, giving a characteristic funnel shape to the parous cervix. Assessment of the postnatal cervix is important in diagnosing retained products of conception (see Secondary postpartum hemorrhage below). Lochia  Lochia is the blood-stained uterine discharge that is comprised of blood and necrotic decidua.  Only the superficial layer of decidua becomes necrotic and is sloughed off. The basal layer adjacent to the myometrium is involved in the regeneration of new endometrium and this regeneration is complete by the third week.  During the first few days after delivery, the lochia is red (lochia rubra); this gradually changes to pink as the endometrium is formed, serous by the second week (lochia serosa) and then ultimately a scanty yellow-white discharge (lochia alba) that lasts for about 1 month.  Persistent red lochia suggests delayed involution that is usually associated with infection or a retained piece of placental tissue.