Next Lesson - The Postpartum Period
Core
Labour is defined as the physiological process by which a fetus is expelled from the uterus to the outside world.
Delivery is the method of expulsion of the fetus, thereby transforming the fetus into a neonate (a newborn baby).
Gravidity is a characteristic of the mother describing the total number of pregnancies this woman has had. This includes the current pregnancy and any previous pregnancies, including miscarriages, stillbirths and terminations.
Parity is another characteristic of the mother describing the total number of births she has delivered with the fetus after 24 weeks of gestation (the length of the pregnancy). If the mother is still pregnant, it does not include the current pregnancy (because she has not given birth yet). It therefore includes any previous children, stillbirths or terminations after 24 weeks (such as for medical reasons).
Overall, labour involves changes to the cervix followed by co-ordinated changes to the uterine myometrium (the muscle layer of the uterus). The whole process aims to achieve two things: dilate the cervix so the fetus can pass through and push the fetus through the birth canal.
The first stage of labour aims to create the birth canal. It is clinically defined as the interval between the onset of labour and full (10cm) dilation of the cervix.
The birth canal is made up of the pelvic bones, the lower uterine segment (the lower, thinner part of the uterus), the cervix, the vagina, the vulva (the outer genitals), the pelvic floor, and the perineum (the area between the vagina and the anus). Together these are called ‘the passage’.
Through this process, the birth canal is created and the fetus’ head descends into it.
There are two phases of the first stage of labour; the latent phase and the active phase.
The latent phase describes the onset of labour with slow cervical dilation (up to about 4 cm) and cervical softening. Its length varies with the parity of the mother (the number of previous births). Multiparous women (those who have given birth before) generally have a shorter latent phase and a shorter overall labour.
The latent phase progresses into the active phase where there is a faster rate of change to the cervix and regular uterine contractions aiming to further dilate the cervix. NICE calls this the established first stage of labour. It is defined as regular contractions with progressive cervical dilation from 4 cm.
The second stage of labour is the time between full dilation of the cervix and delivery of the fetus. The uterine contractions change to become expulsive (pushing contractions that expel the baby), which leads to the descent of the fetus through the birth canal through to delivery. The second stage involves passive elements (descent of the fetus and rotation of the fetal head, reducing the circumference of the fetal head in relation to the birth canal), and active elements (maternal effort to expel the fetus and achieve birth, which is pushing). NICE separates a passive second stage (the cervix is fully dilated but there is no involuntary or active pushing yet) from an active second stage (the baby is visible, or there is involuntary or active pushing, with full dilation).
The third stage of labour starts when the baby is birthed and ends with the complete expulsion of the placenta and the membranes. The uterus achieves this through further contractions. This usually lasts between 5 and 15 minutes. NICE calls it prolonged if it is not complete within 30 minutes of the birth with active management (a routine oxytocin injection, explained below), or within 60 minutes with physiological management (no routine injection).
First Stage of Labour: Initiating Labour
In order to start labour, the myometrium becomes progressively more excitable, through increased communication between smooth muscle cells (via gap junctions, small channels linking neighbouring cells), and the cervix begins to ripen. The exact mechanism behind this is unknown, but it is known that prostaglandins play an important role, which is why they are used to start labour (induction), for example when a pregnancy continues beyond 41 weeks. NICE advises discussing induction from 41+0 weeks, because it may reduce the risks of a longer pregnancy.
Prostaglandins are biologically active lipids which act as local hormones and are produced by the myometrium and the decidua (the endometrium, the lining of the uterus, which has been modified for labour). The production of prostaglandins is controlled by the oestrogen:progesterone ratio, with higher levels of oestrogen promoting prostaglandin release.
Rising levels of prostaglandins stimulate powerful contraction of the smooth muscle and cervical softening. The cervical softening is also helped by oestrogen, which works in combination with prostaglandins to break down the connective tissue. The hormone relaxin may play a part too, but its role in humans is uncertain. Progesterone inhibits contractions.
Oxytocin is a hormone which stimulates and augments uterine contractions. It is secreted from the posterior pituitary gland under control of the hypothalamus. Its action is reduced by high levels of progesterone and relaxin, and by a low number of oxytocin receptors in the myometrium.
The number of oxytocin receptors in the myometrium rises gradually through pregnancy and is much higher by the time of labour. This means the uterus is better able to respond to pulses of oxytocin released from the posterior pituitary.
Oxytocin stimulates the uterus to contract, and also stimulates the decidua and the amnion (the thin membrane lining the sac around the baby) to release prostaglandins, which then go on to stimulate more contractions of the uterus. Uterine contractions have a positive feedback effect on oxytocin release and prostaglandin synthesis.
During pregnancy, the cervix is responsible for retaining the developing fetus. A cervix which cannot stay closed in this way is described as insufficient (older texts say ‘incompetent’). Cervical insufficiency is an imprecise diagnosis. It is suspected in women who have had a second-trimester loss (a miscarriage in the middle months of pregnancy) or a spontaneous preterm birth (a baby born too early). For this reason, the cervix is tough, thick, and high in collagen. Whilst this is good for pregnancy, the cervix needs to soften before labour can begin. This process is called cervical ripening.
Cervical ripening involves reducing the amount of collagen, increasing the amount of glycosaminoglycans (long sugar chains that hold water), and increasing the amount of hyaluronic acid (a water-holding molecule) in the extracellular matrix of the cervix (the material between its cells). This causes reduced aggregation (less clumping together) of the collagen fibres, which makes the cervix softer. This process is triggered by prostaglandin E2 (PGE2) and prostaglandin F2α (PGF2α). Levels of relaxin also increase during pregnancy and may help cervical ripening, but the role of relaxin in humans is uncertain.
Overall, due to cervical ripening, the cervix offers less resistance to the presenting part of the fetus (the part that comes first).
The cervix effaces and dilates as labour progresses. It is fully dilated (10 cm) when the second stage begins.
Effacement is shortening and thinning of the cervix, with reduction in cervical canal length. It is distinct from dilation, which is widening of the cervical opening. Effacement and dilation may occur together. Whether the external os (the opening of the cervix into the vagina) is open or closed is not part of the definition of effacement.
Dilation is the separation of the external cervical os to allow the birth of the baby.
Diagram - Schematic of cervical shortening and thinning (effacement) and opening (dilation). These changes can overlap; the panels do not imply that the external os must remain closed during effacement.
SimpleMed original by Dr. Maddie Swannack
Artificial Induction of Labour
There are a number of methods clinicians use to initiate labour, each with their own specific indications depending on what the clinician thinks the problem may be. We can stimulate the release of prostaglandins through an Artificial Rupture of the Membranes (ARM). Prostaglandin drugs are another option. When the Bishop score is 6 or less, NICE advises vaginal dinoprostone (a tablet, gel or slow-release insert placed in the vagina) or a low dose of oral misoprostol (a tablet taken by mouth). A balloon catheter (a thin tube with an inflatable balloon that opens the cervix) is an alternative if drugs are unsuitable. NICE says the available evidence does not support oral dinoprostone. When the Bishop score is more than 6, NICE advises amniotomy (ARM, described above) together with an infusion of intravenous synthetic oxytocin (Syntocinon). NICE says the available evidence does not support intravenous oxytocin alone for induction. If oxytocin is used, NICE advises offering continuous fetal monitoring, and the mother needs more regular maternal observations. Drugs that block progesterone (anti-progesterone agents), such as mifepristone, are not recommended for induction in a live pregnancy. NICE says the available evidence does not support their use, except in combination with other drugs when the baby has died in the womb (intrauterine fetal death).
First Stage of Labour: Uterine Smooth Muscle Physiology
The myometrium has specific adaptations compared to normal smooth muscle, to help facilitate labour and delivery. A normal muscle contracts and relaxes. The myometrium contracts like any other muscle, but only partially relaxes which causes permanent shortening of the muscle fibres. This process is called contraction and retraction. It helps to expel the fetus through the birth canal, because the uterine cavity becomes progressively smaller.
Contractions start in the two poles of the uterus (the upper parts of the uterus near the entrance of the fallopian tubes). The contractions spread to the uterine fundus (the top of the uterus) and upper segment of the uterus, before moving down to the lower segment. The contractions in the upper segment are more powerful than those in the lower segment.
The contraction force of the myometrium is generated when intracellular concentrations of Ca2+ ions increase following membrane depolarisation and action potential activity. Contractions are made more forceful and frequent by prostaglandins and oxytocin, which promote calcium entry and release and increase uterine muscle excitability.
The Second Stage of Labour: Normal Process of Vaginal Delivery
During the second stage of labour, the fetus undergoes a series of movements in order to help it deliver:
- Head Flexion - the fetal head flexes as the diameter of the occipital portion of the head is smaller than the top of the head
- Head Internal Rotation - the head turns inside the pelvis
- Crowning - the head stretches the perineal muscle and skin
- Extension of Head and Restitution - after the head is crowned the head extends and then turns (external rotation, also called restitution) so that it lines up with the shoulders, which allows delivery of the shoulders
- Rotation and Delivery of the Shoulders
- Delivery of the Rest of the Body - this happens rapidly after the head and shoulders are delivered
The second stage of labour is a high risk part of labour. Both the mother and the fetus are monitored closely to ensure the second stage is progressing well. If there are signs of fetal compromise (the baby is not coping well with labour) or maternal distress, there are a number of options to help deliver the fetus.
An operative vaginal delivery (also called an assisted vaginal delivery) uses an instrument to help the baby out through the vagina. This can be done using forceps on the fetus’ head or through vacuum extraction by placing a suction cup (Ventouse machine) on the fetus’ head. Both carry a risk of injury to the baby and the mother. They are used for suspected fetal compromise in the second stage, or when progress is slow. They are normally only used when the cervix is fully dilated and the baby’s head is engaged (has settled into the pelvis).
A caesarean section can be done electively (planned), in an emergency, or when an operative vaginal delivery has failed. NICE classifies the urgency of caesarean birth from category 1 (immediate threat to the life of the woman or baby) to category 4 (birth timed to suit the woman or the healthcare team). An incision (cut) is made through the skin and then the wall of the uterus, and the baby is delivered through it. Caesarean section has some advantages: it can be done quickly in an emergency with a well-trained and well coordinated team. But it has some risks similar to any other major surgery such as infection, bleeding, damage to nearby structures (bowel and bladder in particular), scar formation, and iatrogenic (caused by treatment) damage to the fetus.
Monitoring of the Fetus During Labour
Monitoring the fetus is like monitoring observations on any other patient, but in fetuses it must be done indirectly.
For women at low risk of complications, NICE advises intermittent auscultation in the established first stage of labour. This means listening to the fetal heart rate at set intervals with a Pinard stethoscope (a small horn-shaped listening tube pressed to the mother’s abdomen) or a handheld Doppler ultrasound (a small ultrasound device). A cardiotocograph (CTG) is offered when there are risk factors. Examples are the use of oxytocin, a raised maternal temperature, or meconium (the baby’s first stool) in the amniotic fluid (the fluid around the baby). A CTG is essentially a belt which is placed on the mother’s abdomen and contains two sensors to measure fetal heart rate and the frequency and duration of uterine contractions. This is a non-invasive method of measuring fetal wellbeing and the results are printed out on a graph to allow for observation over a long period of time.
Other methods of monitoring fetal wellbeing are looking at the mother’s temperature and at the colour and amount of amniotic fluid. Meconium in the fluid can indicate possible fetal compromise. A further method is to make a very small cut on the fetal head and measure the capillary pH of a blood sample. This is called fetal blood sampling (FBS) and it is invasive. NICE has not made a recommendation about FBS because the evidence is limited.
The Three Ps: Problems with Labour
There are three Ps that describe things that can affect delivery of a fetus: the Passage, the Power and the Passenger.
In normal vertex vaginal delivery (head-first birth through the vagina), the fetal head is the relatively rigid presenting part whose fit through the pelvis is important. The shoulders may cause a separate obstruction after the head has delivered. The pelvic inlet (the top opening of the pelvis) usually measures about 11cm from front to back (the anteroposterior diameter). This means that there isn’t a lot of difference between the size of the fetal head and the pelvic inlet, meaning a number of mechanisms are needed to help the fetus be delivered. One of these mechanisms is the stretching of the fibres of levator ani (a pelvic floor muscle) and the thinning of the central portion of the perineum.
If the passage does not increase in size, or if the maternal pelvis is abnormal, normal vaginal delivery can be very difficult.
The frequency of contraction of the myometrium and the power of the contractions contribute to the possibility of normal vaginal delivery. Contractions are counted over 10 minutes. In the first stage of labour, NICE aims for 3 to 4 contractions in 10 minutes when oxytocin is used. In the second stage of labour, the target is the same when oxytocin is used. If there are more than 4 contractions in 10 minutes, the oxytocin dose is reduced or stopped.
If the contractions are not happening quickly enough and labour is slow (delay in the established first stage), NICE advises considering amniotomy (breaking the waters) if the membranes are intact. An obstetrician (a doctor who specialises in pregnancy and birth) then assesses the woman, and an IV infusion of oxytocin can be considered. Its use is discussed with the woman first. If the contractions are happening too quickly (hyperstimulation: the uterus contracts too often or for too long, which can happen with an oxytocin infusion), the first step is to reduce or stop the oxytocin. A tocolytic drug (for example terbutaline, given by injection under the skin) can also be offered to slow contractions, a process called tocolysis.
The position of the fetus can also affect labour.
Lie: The lie of the fetus describes the orientation of the long axis of the fetus (head to feet) in comparison to the long axis of the mother. It can be longitudinal or ‘long’ (the baby lies along the length of the mother, with either the head or the bottom down), oblique (the baby lies at a slant across the mother), or transverse (the baby lies across the mother, side to side). Only long lie babies can be delivered vaginally.
Presentation: The lowest part of the fetus, the part that will come out of the vagina first. The possibilities are cephalic (head first), breech (bottom first), or shoulder presentation. Cephalic is the best presentation for vaginal birth. Breech vaginal delivery is possible but rarer. Shoulder presentation should not be attempted vaginally.
Position: The description of which way the baby is orientated within the presentation. For example, for cephalic presentation, this is which way the baby is looking, or with breech presentation, this is the direction of the sacrum.
- Occipitoanterior - baby is looking at the mother’s spine, their occipital bone is anterior.
- Right Occipitotransverse - baby is looking to the mother’s left side, with the occipital bone towards the maternal right thigh.
- Left Occipitotransverse - baby is looking to the mother’s right side, with the occipital bone towards the maternal left thigh.
- Occipitoposterior - baby is looking out the front, ‘back to back’.
Occipitoanterior is the easiest position to deliver the baby vaginally, but all are possible. Most babies in an occipitotransverse (looking to the side) or occipitoposterior position rotate on their own to the occipitoanterior position as labour progresses.
Flexion: As described earlier, when the fetus is being born, it undergoes a number of movements, one of which is neck flexion. The degree of neck flexion is important, because increased flexion decreases the diameter of the head and so encourages easier vaginal birth.
The position that is ideal for delivery is occipitoanterior with lots of neck flexion (chin on chest), as this creates the smallest head diameter. If the neck is not flexed this much, or in the occipitoposterior position, the diameter is larger and this can slow the first or second stages of labour.
Moulding: The sutures in the fetal skull also allow for moulding, when the suture lines overlap and shuffle to allow the circumference of the head to decrease. This means that babies just after vaginal birth may not have a normal shaped head.
After the baby is born, there is a massive reduction in the size of the uterus, but the contractions continue. This causes squeezing of the placenta which causes it to separate from the uterus and descend.
There are a number of mechanisms which act to reduce the amount of blood lost after the delivery of the placenta; the failure of any of these mechanisms can lead to a postpartum haemorrhage.
The powerful contraction and retraction of the uterus continue after delivery of the placenta, and compress the blood vessels that supplied the placenta.
The third stage can be ‘actively managed’, which is a package of care. The cord is clamped and cut, and the placenta is delivered by controlled cord traction (gentle traction on the cord once the placenta has started to separate). An intramuscular injection of oxytocin is given first, usually into the thigh, straight after the birth of the baby. This encourages uterine contraction, which decreases the chance of postpartum haemorrhage. NICE advises telling women that active management is associated with a lower risk of postpartum haemorrhage and of needing a blood transfusion. Some women choose physiological management instead. This means no routine injection, and the placenta is delivered by the woman’s own effort. Care should change to active management if there is haemorrhage or if the placenta is not delivered within 1 hour.
For information on changes that occur to the fetal circulation after birth, see the article on the embryology of the cardiovascular system.
Edited by: Dr. Maddie Swannack
Reviewed by: Dr. Thomas Burnell and Dr. Marcus Judge
- 4032
