Definition of labour.
Labour orparturitionis the process whereby the products of conception are expelled from the uterine cavity after the 24th week of gestation.
The three stages of labour.
- The first stage commences with the onset of labour and terminates when the cervix has reached full dilatation and is no longer palpable.
- The second stage or stage of expulsion begins at full cervical dilatation and ends with expulsion of the fetus.
- The third stage or placental stage begins with the delivery of the child and ends with the expulsion of the placenta.
The Mechanism of Labour
The mechanism of labour relates to the manoeuvres undertaken by the fetus during childbirth.
Onset of labour.
The onset of The onset of labour is defined as the time of onset of regular, painful uterine contractions, which produce progressive effacement and dilatation of the cervix. It is often difficult to be certain of the exact time of onset of labour because of the occurrence of ?false labour? where the onset of painful contractions is not associated with progressive dilatation of the cervix.
The clinical signs of the onset of labour include:
- The onset of regular, usually painful contractions that produce progressive cervical dilatation
- The exhibition of a vaginal show ? the passage of blood stained mucus.
- Rupture of the fetal membranes ? this is variable and may occur at the time of onset of contractions or it may be delayed until the delivery of the fetus.
Making a decision about the time of onset of labour has important implications for the subsequent management of labour. An assumption that labour is abnormally prolonged may result from an erroneous decision as to the time of onset of labour - see active management of labour.
The initiation of labour.
Despite extensive research, the mechanism of the onset of labour remains uncertain.
There are many factors that change at the time of the onset of labour. Furthermore, the inhibition and promotion of certain factors can both delay or accelerate the process of parturition. It is unlikely that any one factor is sufficient to provide an explanation for the onset of labour as intervention at any one of several biochemical points can either stimulate or delay it.
It is likely that there is a cascade of events regulated and controlled by the fetoplacental unit.
During pregnancy, uterine activity is present but is minimal. At the end of gestation, there is a gradual downregulation of those factors that keep the uterus and cervix quiescent and an upregulation of procontractile influences.
- At term, the fetus increases its production of cortisol and this cortisol reduces the production of placental progesterone and increases the production of oestrone and oestradiol.
- Progesterone suppresses uterine activity and oestradiol increases it.
- These changes also result in increased production of prostaglandins by the placenta and thus a further increase in myometrial activity. These changes also stimulate oxytocin release, which also enhances myometrial activity. Other hormones produced in the placenta also act directly or indirectly on the myometrium, such as relaxin, activin A, follistatin, hCG and corticotrophin-releasing hormone (CRH).
The cervix contains myocytes and fibroblasts and serves to contain the products of conception. Towards term, the cervix becomes softened as there is a decrease in the amount of collagen and an increase in proteolytic enzyme activity. Increased production of hyaluronic acid reduces the affinity of fibronectin for collagen and, in conjunction with the affinity of hyaluronic acid for water, there is a conse quent softening and ripening of the cervix.
Increasing cervical compliance allows progression of labour with reduced intrauterine pressure. The cervix also contracts during labour up to 3?4 cm dilatation but, in the active phase of labour, cervical dilatation occurs secondary to uterine contractions alone. In other words, the cervix is passively stretched by the increasing strength of the uterine contractions.
The Powers - Uterine Activity In Labour
The uterus exhibits infrequent, low-intensity contractions throughout pregnancy. As full term approaches, uterine activity increases in both the Frequency and strength of contractions. With the onset of labour, intrauterine pressures rise to 20?30 mmHg during contractions that occur every 10?15 minutes and last approximately 30?40 seconds. Normal resting tonus in labour starts at around 10 mmHg and increases slightly during the course of labour. Contractions increase in intensity to reach pressures of 50 mmHg ? around 5 kPa ? in terms of active pressure in the first stage of labour (Figure 1).
Uterine contractions reach pressures of 50 mmHg (6.5 kPa) with first stage of labour. Contractions become painful when amniotic pressure exceeds 25 mmHg (3.2 kPa).
(From Symonds E M: Essential Obstetrics and Gynaecology, 4th ed. Edinburgh, C hurchill Livingstone, 2003, p 153)
In the second stage, with the additional effect of voluntary expulsive efforts, intrauterine pressure may rise to 100 mmHg. Throughout labour, contractions produce effacement (Figure 2) and dilatation of the cervix as the result of shortening of myometrial fibres in the upper uterine segment and stretching and thinning of the lower uterine segment. This process is known asretraction. The lower segment becomes elongated and thinned as labour progresses and the junction between the upper and lower segment rises in the abdomen. Where labour becomes obstructed, the junction of the upper and lower segments may become visible at the level of the umbilicus; this is known as a retraction ring.
Contractions are initiated by a pacemaker in the left uterine cornua and spread downwards through the myometrium. Contractions occur first in the fundus of the uterus, where they are stronger and last longer than in the lower segment. This phenomenon is known as fundal dominance and is essential to progressive effacement and dilatation of the cervix. As the uterus and the round ligaments contract, the axis of the uterus appears to straighten, pulling the longitudinal axis of the fetus towards the anterior abdominal wall in line with the inlet of the true pelvis.
The realignment of the uterine axis promotes descent of the presenting part as the fetus is pushed directly downwards into the pelvic cavity.
Figure 2. Cervical Effacement.The Passages
The female pelvis has a wide birth canal and wide pubic arch (Figure 3). Because of softening of the sacroiliac ligaments and the pubic symphysis, some expansion of the pelvic cavity can occur. The soft tissues also become more distensible than in the non-pregnant state and substantial distension of the pelvic floor and vaginal orifice occurs during the descent and birth of the head. This commonly results in tearing of the perineum and of the vaginal walls and sometimes in tearing and disruption of the external anal sphincter.
Figure 3. The female pelvis
The Second Stage of Labour
The Mechanism Of Labour
The head normally engages in the pelvis in the transverse position and the passage of the head and trunk through the pelvis follows a well-defined pattern (Figure 4).
The process of normal labour involves the adaptation of the fetal head to the various segments and diameters of the maternal pelvis and the following processes occur. Descent occurs throughout labour and is both a feature and a pre-re quisite for the birth of the baby. Engagement of the head normally occurs before the onset of labour in the primigravid woman but may not occur until labour is well established in a multipara. Descent of the head provides a measure of the progress of labour.
Flexion of the head occurs as it descends and meets the pelvic floor, bringing the chin into contact with the fetal thorax. Flexion produces a smaller diameter of presentation, changing from the occipito-posterior diameter, when the head is deflexed, to the suboccipito-bregmatic diameter when the head is fully flexed.
Figure 5. Head descending.
Internal rotation: The head rotates as it reaches the pelvic floor and the occiput normally rotates anteriorly from the lateral position towards the pubic symphysis (Figure 6). Occasionally, it rotates posteriorly towards the hollow of the sacrum and the head may then deliver as a face-to-pubes delivery.
Figure 6. Rotation of head to occipito-anterior position.
Extension: The acutely flexed head descends to distend the pelvic floor and the vulva, and the base of the occiput comes into contact with the inferior rami of the pubis. The head now extends until it is delivered. Maximal distension of the perineum and introitus accompanies the final expulsion of the head, a process that is known as crowning (Figure 7).
Figure 7. Delivery of the head.
Restitution: Following delivery of the head, it rotates back to be in line with its normal relationship to the fetal shoulders (Figure 8).
Figure 8. External rotation of the head allowing delivery of the shoulders.
External rotation: When the shoulders reach the pelvic floor, they rotate into the antero-posterior diameter of the pelvis. This is accompanied by rotation of the fetal head so that the face looks laterally at the maternal thigh.
Delivery of the shoulders Figures 9 and 10.
Figure 9. Delivery of the Anterior Shoulder.
Figure 10. Delivery of the Posterior Shoulder.
Final expulsion of the trunk occurs following delivery of the shoulders. The anterior shoulder is delivered first by traction posteriorly on the fetal head so that the shoulder emerges under the pubic arch. The posterior shoulder is delivered by lifting the head anteriorly over the perineum and this is followed by rapid delivery of the remainder of the trunk and the lower limbs.
The Valsalva pushing technique (the glottis is closed so that the mother pushes without expelling air) is used routinely in the second stage of labour in many countries, and it is accepted as standard obstetric management in Turkey.
The purpose of a study in Turkey was to determine the effects of pushing techniques on mother and fetus in birth in this setting. This randomized study was conducted between July 2003 and June 2004 in Bakirkoy Maternity and Children's Teaching Hospital in Istanbul, Turkey. One hundred low-risk primiparas between 38 and 42 weeks' gestation, who expected a spontaneous vaginal delivery, were randomized to either a spontaneous pushing group or a Valsalva-type pushing group. Perineal tears, postpartum haemorrhage, and haemoglobin levels were evaluated in mothers; and umbilical artery pH, Po(2) (mmHg), and Pco(2) (mmHg) levels and Apgar scores at 1 and 5 minutes were evaluated in newborns in both groups. No significant differences were found between the two groups in their demographics, incidence of nonreassuring fetal surveillance patterns, or use of oxytocin. The second stage of labour and duration of the expulsion phase were significantly longer with Valsalva-type pushing. Differences in the incidence of episiotomy, perineal tears, or postpartum hemorrhage were not significant between the groups. The baby fared better with spontaneous pushing, with higher 1- and 5-minute Apgar scores, and higher umbilical cord pH and Po(2) levels. After the birth, women expressed greater satisfaction with spontaneous pushing. It was concluded that educating women about the spontaneous pushing technique in the first stage of labour and providing support for spontaneous pushing in the second stage result in a shorter second stage without interventions and in improved newborn outcomes. Women also stated that they pushed more effectively with the spontaneous pushing technique.0801
The Third Stage of Labour
The third stage of labour starts with the completed expulsion of the baby and ends with the delivery of the placenta and membranes.
Immediately after delivery of the baby, the placenta is still attached inside the uterus. Some time after delivery, the placenta will detach from the uterus and then be expelled. This process is called the "3rd stage of labour" and may take just a few minutes or as long as an hour.
Signs that the placenta is beginning to separate include:
A sudden gush of blood
Lengthening of the visible portion of the umbilical cord.
The uterus, which is usually soft and flat immediately after delivery, becomes round and firm.
The uterus, the top of which is usually about half-way between the pubic bone and the umbilicus, seems to enlarge and approach the umbilicus.
Immediately after the delivery of the baby, uterine contractions stop and labour pains go away. As the placenta separates, the woman will again feel painful uterine cramps. As the placenta descends through the birth canal, she will again feel the urge to bear down and will push out the placenta.
Figure 11. Separation of the placenta from the uterus.
Figure 12 The placenta with the membranes (bag) after delivery.
Promoting normal vaginal delivery.
As As operative vaginal delivery can be associated with maternal and neonatal morbidity, strategies that reduce the risk of operative vaginal delivery should be used.
Continuous support for women during childbirth can reduce the incidence of operative vaginal delivery, particularly when the carer was not a member of staff.0302 Use of any upright or lateral position, compared with supine or lithotomy positions is associated with a reduction in assisted deliveries.0402 Epidural analgesia appears to be effective in reducing pain during labour. However, women who use this form of pain relief are at increased risk of having an instrumental delivery. Epidural analgesia had no statistically significant impact on the risk of caesarean section, maternal satisfaction with pain relief and long-term backache and does not appear to have an immediate effect on neonatal status as determined by Apgar scores.0502
Using a partogram leads to fewer operative births and less use of oxytocin.0503 An oxytocin infusion may reduce the high rate of operative delivery in primigravidae associated with epidural analgesia provided that the fetal occiput is in an anterior position at the onset of the second stage of labour8901 Extreme caution should be taken before using oxytocin for the second stage in multiparous women. Each woman should be assessed individually for the management of the second stage.
In the study by Fitzpatrick et al,0202 rates of instrumental delivery were similar following immediate and delayed pushing, in association with epidural analgesia. 0303 The , however, sites a recent meta-analysis demonstrated that primiparous women with epidurals were likely to have fewer rotational or mid-cavity operative interventions when pushing was delayed for 1?2 hours or until they had a strong urge to push.
There is no difference between the rates of operative vaginal delivery for combined spinal-epidural and epidural techniques.0304
Photographs of normal deliveries are available on birthdiaries.com
A video of a normal delivery can be seen on http://livehumanbirthvideo.com/free-giving-birth-video-7/