Formation of the placenta and the third stage of labour.
I will discuss first what the placenta is, and how it is formed. I will then write about the physiology of the third stage of labour, and apply this to a client from my clinical practice.
The placenta links closely with the circulation of the mother to carry out functions that fetus in unable to perform for itself during intra-uterine life (Bennett & Brown 1996). The placenta is a haemochorial villous organ, meaning it has a villous structure, and the chorion, or trophoblast, is in direct contact with maternal blood in the intervillous space (Fox 1991).
Development of the placenta.
The name given to the fertilised ovum is the zygote. Within a few hours of conception, still within the fallopian tube, it undergoes a series of cleavage divisions, known as mitosis. The nucleus of the cell divides into two, so that two new cells are formed, each with an individual set of chromosomes. The zygote continually reproduces until it resembles a berry. This is called the morula. The cell division is aided by progesterone from the corpus luteum, which together with oestrogen, is preparing the uterine endometrium to receive a fertilised ovum (Verralls 1993). The morula is still inside the zona pellucida, which is a protective outer casing, and is supported by its own cytoplasm, which contains progesterone. At the end of the first week after fertilisation, the morula lies adjacent to the uterine endometrium, which is in its secretory phase. It now begins to implant into the endometrium. Also by the end of the first week, some inner cells in the morula begin to disintegrate, leaving a cavity which fills with fluid. The cell is now called a blastocyst (Verralls 1993). The blastocyst consists of 34-64 cells, and utilises metabolic substrates from endometrial fluids, for around another 24 hours, before the outer cells digest their way out of the zona pellucida and come into direct contact with the uterine epithelium (McNabb 1997). At this point, the outer rim of cells begins to synthesize a glycoprotein hormone called human chorionic gonadotrophin (hCG), that is very similar to luteinising hormone (LH). Because of this similarity, the hCG acts on the LH receptors in thecal and granulosa cells of the corpus luteum. This provides a continuous stimulus for increased secretion of progesterone. The hormones maintain the endometrium in an optimum state for implantation of the pregnancy (McNabb 1997). By this point, the pregnancy may be able to be detected using a sensitive home pregnancy testing kit which is designed to detect the presence of hCG in the urine.
The blastocyst consists of:
i) an inner cell mass, which will develop to form the fetus and the placental membrane known as the amnion.
ii) The trophoblast. This is the outer layer of single cells and from this layer, small root-like structures, known as the primitive chorionic villi start to grow. Some will form the placenta and some will atrophy to form the chorionic membrane, which surrounds the amniotic sac and lines the uterus. This stage of development is reached 7-10 days after conception (Verralls 1993).
By day 10 after conception, the blastocyst is completely buried in the endometrium, which is now named the decidua, as it is shed following the birth (Stables 1999). The trophoblast cells rapidly differentiate into two layers that form the chorion and the fetal portion of the placenta. The chorion forms the wall of the chorionic sac within which the inner cell mass (which will become the fetus) and the amniotic and yolk sacs are suspended by the connecting stalk. Trophoblast cells in contact with the decidua become cuboidal (McNabb 1997). These are known as cytotrophoblast cells and they contain a single nucleus and a well-defined plasma membrane (McNabb 1997). They continually divide to form an outer layer of multinucleated syncytiotrophoblast (or plasmoditotrophoblast) tissue, which ceases to have distinguishable cell membranes (McNabb 1997). By day 14, the primitive chorionic villi, which are now finger like projections, have developed from the trophoblast, and continue to grow, until they cover the whole surface of the chorion by the end of the 3rd week. At the same time, embryonic blood vessels are beginning to form in the mesoderm of the inner cell mass (Verralls 1993).
Each chorionic villus is composed of a single layer of cells, called the layer of Langhan’s, which is surrounded by syncytiotrophoblastic cells. The spaces between them as they erode more and more deeply into the decidua are called choriodecidual spaces (Bennett & Brown 1999). As the villi erode the walls of the maternal blood vessels, they open up and form a lake of maternal blood in which they float. The opened blood vessels are known as sinuses (Bennett & Brown 1999).
The maternal blood circulates slowly, enabling the villi to absorb food and oxygen, and to excrete waste. These are known as the nutritive villi of the placenta. Some other villi are more deeply attached to the decidua, and are called anchoring villi (Bennett & Brown 1999).
During the 3rd week, branching of the chorionic villi occurs. These branches are called secondary primitive chorionic villi, and blood vessels form within them (Verralls 1993). Tertiary chorionic villi are so called when the blood vessels have formed and become attached to the embryonic blood vessels in the body stalk. The vessels in the stalk develop to form two arteries and one umbilical vein for the fetus (Verralls 1993).
In the uterus, the decidua is now differentiated into 3 areas:
i) Decidua basalis lies below where the chorionic villi first embedded
ii) Decidua capsularis lies below the embryonic sac
iii) Decidua vera (or parietalis) lines the remainder of the uterine cavity.
As the fetus grows, the decidua capsularis bulges into the uterine cavity, and eventually fuses with the decidua vera, obliterating the uterine cavity. By 22 weeks, the decidua capsularis has degenerated and disappeared (Stables 1999).
The entire surface of the chorionic sac is covered by chorionic villi until the 8th week. As the sac grows, the chorionic villi associated with the decidua capsularis become compressed. Their blood supply is reduced and they degenerate leaving a bare area, the chorionic laeve, which becomes the chorionic membrane. The chorionic villi of the decidua basalis increase, branch and enlarge rapidly to form the chorion frondosum, forming the fetal part of the placenta. By 16 weeks, the placenta has reached its full thickness and no new cotyledons (lobes) or stem villi develop (Stables 1999). After the 20th week, the placenta continues to increase in circumference, until at full term it is about 23cm in diameter, a round flat organ about 2cm thick in the centre, but thinner at the circumference (Verralls 1993).
The third stage of labour is the period following delivery of the baby, until expulsion of the placenta and membranes. The process is concerned with separation and delivery of the placenta and membranes, and the control of bleeding from the placental site (Morrin 1999).
Separation of the placenta usually begins with the contraction that delivers the baby’s body (Verralls 1993). The placental site diminishes in size and the placenta is compressed so that blood in the intervillous spaces is forced back into the spongy layers of the decidua. Retraction of the oblique muscle fibres of the uterus, constricts the blood vessels supplying the placenta so that the blood cannot drain into the maternal vascular tree (Stables 1999). With the next contraction after the birth of the baby, these uterine vessels which are now tense and congested, rupture and bleed into the decidua, causing a blood clot, and also tearing the layer of perforated cells between the decidua and the myometrium, which is known as Nitabuch’s layer (Verralls 1993).
The non-elastic placenta is detached from the uterine wall. Separation normally begins from the centre of the placenta, so that no blood escapes, and a retroplacental clot forms. The weight of the placenta strips the membranes off the uterine wall, and the placenta descends fetal side first, enclosing the blood clot in a complete bag of membranes, into the vagina and out of the body. This method was described by Schultze, and is known as the Schultze method of separation (Stables 1999). There is another method of placental separation, which begins unevenly at one of the lateral borders of the placenta. Blood escapes from behind the placenta, so that there is no retroplacental clot. The placenta folds in on itself, and descends maternal side first (Stables 1999). There is a heavier blood loss associated with this method of separation than with the Schultze method. This method of separation is known as the Matthews Duncan method, and because of the heavier blood loss, has earned itself the nickname of the ‘dirty Duncan’!
Once separation is complete, the uterus contracts strongly, and the placenta and membranes fall into the lower uterine segment and then into the vagina. At least 500ml of blood per minute flows through the placental site, so this must be stopped in seconds to avoid a serious post-partum haemorrhage (Stables 1999).
There are four factors involved in this process:
i) the tortuous uterine blood vessels are surrounded by the oblique muscle fibres, which retract and act as living ligatures
ii) once the placenta has left the upper segment, a vigorous contraction brings the walls of the uterus into apposition, applying pressure to the placental site.
iii) There is a transitory increase in the activity of the blood coagulation system during and immediately after placental separation, so that clot formation in the torn blood vessels is maximised. A fibrin mesh rapidly covers the placental site.
iv) Breast feeding the baby will help achieve placental separation by causing a release of oxytocin from the maternal posterior pituitary gland.
There are two ways of managing the third stage of labour. There is active management using oxytocic drugs to enhance uterine contraction and retraction, and thereby placental separation (Morrin 1997). The umbilical cord must be clamped and cut before delivery of the placenta, so that the baby does not receive a wave of maternal blood through the umbilical cord when the placenta separates. This is combined with controlled traction on the umbilical cord by the midwife, which speeds up the process of separation and delivery of the placenta, and reduces blood loss. The oxytocic drug of choice is Syntometrine, 1ml of which is given intra-muscularly, usually with the birth of the anterior shoulder (Morrin 1997). Syntometrine contains a combination of ergometrine 500ug and oxytocin 5units in 1ml. The oxytocin will induce strong rhythmic contraction of the muscle fibres of the upper segment of the uterus within 2-3 minutes of its administration (Morrin 1997). Its effect lasts for approximately 5-15 minutes (Baskett 1991). It is designed to initiate strong uterine action, which is sustained by the action of the ergometrine (Morrin 1997). The midwife performs controlled cord traction to deliver the placenta and membranes. As soon as the uterus contracts, the midwife places her left hand just above the symphysis pubis with the palm facing towards the umbilicus. The hand is level with the junction of the upper and lower uterine segments and is used to push the uterus upwards to prevent uterine prolapse. The midwife grasps the cord with the right hand and applies steady sustained traction on the cord in a downward direction. When the placenta is visible, traction is exerted in an upward direction following the curve of the birth canal. The placenta should emerge from the vagina, and if the membranes are not completely separated, they should be eased out gently, to prevent tearing (Morrin 1997).
There is also physiological management of the third stage, where the placenta and membranes separate without the use of oxytocic drugs, and expulsion is achieved by maternal effort. If the baby does not need resuscitation or immediate examination, the umbilical cord is left unclamped (Inch 1985). Clamping of the cord will cause fetal blood to be trapped within the placenta, and this produces a counterpressure that impedes the physiological process (Morrin 1997). With physiological third stage, the midwife must observe for signs of placental separation and descent. She must also place her hand on the fundus to ensure it is contracting, and not relaxing and filling with blood. The following signs indicate separation and descent of the placenta:
i) as placental separation occurs, a small amount of fresh blood loss occurs
ii) the uterus becomes smaller, narrower and rounder. The fundus rises in the abdomen and is harder and more freely mobile. This indicates that the placenta is in the lower uterine segment.
iii) The umbilical cord lengthens.
Once separation and descent have occurred, the placenta and membranes can be expelled (Morrin 1997).
The woman may feel an urge to push, and can push the placenta out without any prompting. If the woman cannot expel the placenta, fundal pressure can be used. The midwife pushes downwards and backwards on the fundus of the contracted uterus with her left hand. The uterus pushes against the placenta, and the placenta is delivered. This should never be performed by the unskilled, as it can cause considerable pain and bruising, and predispose the woman to post-partum haemorrhage (McDonald 1999).
When a midwife is assessing a mother during the third stage, there are a number of things she can do to ensure that things are progressing normally. It depends on what type of third stage management the mother has chosen, as to the type of observations the midwife will carry out. If the woman is having a managed third stage, the oxytocic drug should have been administered with the delivery of the baby’s anterior shoulder (McDonald 1999). The midwife must make several checks to ensure normal progress. These are:
i) that an oxytocic drug has been administered,
ii) that it has had time to act,
iii) that the uterus is well contracted,
iv) that counter-traction is applied,
v) that signs of placental separation and descent are present (McDonald 1999).
It is important not to manipulate the uterus, as this may precipitate incoordinate action (McDonald 1999). The midwife should only apply controlled cord traction (CCT) when all the above checks have been made, and she is sure that everything is progressing normally. If CCT is applied too early, or without feeling uterine contraction, then uterine prolapse may occur (McDonald 1999).
If the woman is having a physiological third stage, the cord is usually left unclamped, unless the baby needs resuscitation. The midwife must observe the mother, and wait for signs of separation and descent. She must watch for any signs of relaxation of the uterus and excessive blood loss, as this is a sign of post-partum haemorrhage. She should place her hand on the fundus to ensure it is contracted. Again, the uterus must not be manipulated in any way, as this can interfere with uterine contraction. If the placenta cannot be delivered by maternal effort, then fundal pressure can be applied (Morrin 1997).
It is important in both cases that the midwife observes for signs of excessive blood loss, as if more than 500ml is lost, it is considered to be a post-partum haemorrhage.
I will now discuss how I assessed the progression of the third stage on a client from my clinical practice. In accordance with UKCC guidelines, I have changed the clients’ name, and any identifiable details.
Joanne was a 20-year-old primigravida, who was admitted to the labour ward at 38 weeks gestation with regular contracting pains. Her membranes had not ruptured when she arrived. I looked at her antenatal notes, and she appeared to be a healthy young woman, who had had a trouble free pregnancy. Her haemoglobin level was 10.2 and her blood group A rhesus D positive. Upon vaginal examination at 06:20, Joanne’s cervix was fully effaced and 4cm dilated. The presenting part appeared to be 2cm above the ischial spines. Although she seemed quite distressed by the pain, Joanne declined any form of pain relief. She was now contracting twice in ten minutes, for about 30 seconds at a time. We discussed what Joanne would like to do when it came to the third stage, and she expressed a preference for a managed third stage, so I drew up 1ml of Syntometrine, ready for when the baby was delivered.
Everything seemed to move very quickly from this point. At 07:05, Joanne expressed an urge to push. Upon vaginal examination, her cervix was 8cm dilated. At this point the supervising midwife performed an artificial rupture of the membranes of the amniotic sac. I encouraged Joanne not to push yet, and helped her to breathe through the pain. At 07:40, Joanne asked to be examined again, and her cervix was fully dilated. Joanne pushed extremely well throughout the second stage of her labour, and I delivered her of an 8lb 5oz boy at 08:23. The supervising midwife administered 1ml of Syntometrine intra-muscularly when the baby’s anterior shoulder was born. I clamped the cord in two places, and cut between the clamps. The baby did not need resuscitating, and was wrapped up and given to Joanne to cuddle. I placed my left hand on the fundus of Joanne’s uterus and found it to be well contracted. At 08:28, I noticed a trickle of fresh blood from Joanne’s vagina, and the umbilical cord appeared to lengthen, indicating that separation and descent had occurred. I applied supra-pubic pressure with my left hand to just above the symphysis pubis, exerting pressure in an upward direction to prevent prolapse. With my right hand I grasped the cord, and applied traction in a downward direction, in line with the curve of the birth canal. I maintained the traction, and could feel the placenta delivering. At 08:30, the placenta and membranes were delivered, and after careful inspection, appeared to be complete. Unfortunately, Joanne had sustained a 2nd degree tear to her perineum, and it was sutured by a senior house officer. Her blood loss following delivery was minimal, estimated to be 150ml, and she was transferred to the post-natal ward at around 11am.
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