The First Stage of Labour

Chapter 17 The First Stage of Labour


Normal labour is spontaneous in onset between 37 and 42 weeks’ gestation with the fetus presenting by the vertex, culminating in a healthy mother and baby.





Phases of the first stage


These are described in Box 17.1.



Box 17.1 Phases of the first stage of labour



The latent phase




• Precedes the active first stage of labour


• May last 6–8 hours in first-time mothers


• The cervix dilates from 0 cm to 3–4 cm


• The cervical canal shortens from 3 cm long to less than 0.5 cm



The active first stage




• Begins when the cervix is 3–4 cm dilated


• In the presence of rhythmic contractions, is complete when the cervix is fully dilated (10 cm)


• Usually completed within 6–12 hours



The transitional phase




• The cervix expands from around 8 cm dilated until it is fully dilated (or until the expulsive contractions during the second stage are felt by the woman)


• There is often a brief lull in the intensity of uterine activity at this time



The onset of spontaneous normal labour


The onset of labour appears to be initiated by a combination of hormonal and mechanical factors.



image Levels of maternal oestrogen rise sharply during the last weeks of pregnancy, resulting in changes that overcome the inhibiting effects of progesterone.


image High levels of oestrogens cause uterine muscle fibres to display oxytocic receptors and form gap junctions with each other.


image Oestrogen also stimulates the placenta to release prostaglandins that induce a production of enzymes that will digest collagen in the cervix, helping it to soften.


It is thought that both fetal and placental factors are involved in the process. Uterine activity may also result from mechanical stimulation of the uterus and cervix, brought about by overstretching or pressure from a presenting part that is well applied to the cervix.


When the woman is in labour, contractions will often be accompanied or preceded by a blood-stained mucoid ‘show’; this results from the operculum, which formed the cervical plug during pregnancy, being lost when the cervix dilates.


Occasionally the membranes will rupture; this should always be reported to the midwife, who will check that there are no changes in the fetal heart rate and that meconium is not present in the liquor.




Spurious labour


Many women experience contractions before the onset of labour; these may be painful and may even be regular for a time, causing a woman to think that labour has started. The two features of true labour that are absent here are:



image effacement of the cervix


image dilatation of the cervix (see below).



Physiological processes



Uterine action



Fundal dominance (Fig. 17.1)


Each uterine contraction starts in the fundus near one of the cornua and spreads across and downwards. The contraction lasts longest in the fundus where it is also most intense, but the peak is reached simultaneously over the whole uterus and the contraction fades from all parts together and is weakest in the lower segment.


image

Fig. 17.1: Fundal dominance during uterine contractions.



Polarity


Polarity is the neuromuscular harmony that prevails between the two poles or segments of the uterus throughout labour. During each uterine contraction:



image the upper pole contracts strongly and retracts to expel the fetus


image the lower pole contracts slightly and dilates to allow expulsion to take place.


If polarity is disorganised, then the progress of labour is inhibited.



Contraction and retraction


Retraction is when muscle fibres retain some of the shortening of the contraction instead of becoming completely relaxed (Fig. 17.2). It assists in the progressive expulsion of the fetus; the upper segment of the uterus becomes gradually shorter and thicker and its cavity diminishes.


image

Fig. 17.2: How uterine muscle retains some shortening after each contraction.


Before labour becomes established, uterine contractions may occur every 15–20 minutes and may last for about 30 seconds; they may be imperceptible to the mother. By the end of the first stage they occur at 2–3-minute intervals, last for 50–60 seconds and are very powerful.



Formation of upper and lower uterine segments


By the end of pregnancy, the body of the uterus is described as having divided into two anatomically distinct segments:



image The upper uterine segment is formed from the body of the uterus.


image The lower uterine segment is formed from the isthmus and the cervix, and is about 8–10 cm in length.


The muscle content reduces from the fundus to the cervix, where it is thinner. When labour begins, the retracted longitudinal fibres in the upper segment pull on the lower segment, causing it to stretch; this is aided by the force applied by the descending presenting part. A ridge forms between the upper and lower uterine segments; this is known as the physiological retraction ring.



Cervical effacement


‘Effacement’ refers to the inclusion of the cervical canal into the lower uterine segment (Fig. 17.3).


image

Fig. 17.3: (A) The cervix before effacement. (B) The cervix after effacement. The cervical canal is now part of the lower uterine segment.


Effacement may occur late in pregnancy, or it may not take place until labour begins.



image In the nulliparous woman, the cervix will not usually dilate until effacement is complete.


image In the parous woman, effacement and dilatation may occur simultaneously and a small canal may be felt in early labour. This is often referred to as a ‘multips os’.



Cervical dilatation


Dilatation of the cervix is the process of enlargement of the os uteri from a tightly closed aperture to an opening large enough to permit passage of the fetal head.



image Dilatation is measured in centimetres and full dilatation at term equates to about 10 cm.


image Dilatation occurs as a result of uterine action and the counterpressure applied by either the intact bag of membranes or the presenting part, or both.


image A well-flexed fetal head closely applied to the cervix favours efficient dilatation.


image Pressure applied evenly to the cervix causes the uterine fundus to respond by contraction and retraction.



Mechanical factors



Formation of the forewaters


As the lower uterine segment forms and stretches, the chorion becomes detached from it; the increased intrauterine pressure causes this loosened part of the sac of fluid to bulge downwards into the internal os. The well-flexed head fits snugly into the cervix and cuts off the fluid in front of the head, the ‘forewaters’, from that which surrounds the body, the ‘hindwaters’.



General fluid pressure


While the membranes remain intact, the pressure of the uterine contractions is exerted on the fluid and, as fluid is not compressible, the pressure is equalised throughout the uterus and over the fetal body; it is known as ‘general fluid pressure’. Preserving the integrity of the membranes optimises the oxygen supply to the fetus and helps to prevent intrauterine and fetal infection.



Rupture of the membranes




image The optimum physiological time for the membranes to rupture spontaneously is at the end of the first stage of labour after the cervix becomes fully dilated and no longer supports the bag of forewaters.


image The uterine contractions are also applying increasing force at this time.


image Membranes may sometimes rupture days before labour begins or during the first stage.


image If there are no other signs of labour but the history of ruptured membranes is convincing or obvious liquor is draining, then digital examination should be avoided owing to an increased risk of ascending infection.


image If the diagnosis is not obvious, then one sterile speculum examination should be performed to try to visualise pooling of liquor in the posterior fornix; endocervical swabs may also be taken at this time.


image The majority of women will labour spontaneously within 48 hours. After 48 hours an obstetrician may consider augmentation of labour.


image Women with prelabour ruptured membranes should have their temperature recorded and be monitored for signs of fetal compromise associated with infection.


image Occasionally, the membranes do not rupture, even in the second stage, and appear at the vulva as a bulging sac covering the fetal head as it is born; this is known as the ‘caul’.



Fetal axis pressure


During each contraction the uterus rises forward and the force of the fundal contraction is transmitted to the upper pole of the fetus and down the long axis of the fetus, and applied by the presenting part to the cervix. This is known as ‘fetal axis pressure’ and becomes much more significant after rupture of the membranes and during the second stage of labour.



Observations and care in labour



Maternal wellbeing


Women must be enabled to be in control throughout labour, with the midwife providing evidence-based information, listening to the woman and ensuring consent is given before any intervention.



Past history and reaction to labour


Factors of particular relevance at the onset of labour are listed in Box 17.2.



Box 17.2 Important factors in the history at the onset of labour




• The birth plan – whatever choices the woman makes, she must be the focus of the care, and should be able to feel she is in control of what is happening to her and able to make decisions about her care


• Parity and age


• Character and outcomes of previous labours


• Weights and condition of previous babies


• Attendance at any specialist clinics


• Any known problems – social or physical


• Blood results, including Rhesus isoimmunisation and haemoglobin (Hb)

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Related posts:

  1. The Placenta
  2. Prolonged Pregnancy and Disorders of Uterine Action
  3. The Third Stage of Labour
  4. Problems of Pregnancy

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Jul 11, 2016 | Posted by in MIDWIFERY | Comments Off on The First Stage of Labour

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