Pregnancy and Childbirth
5.3a The physiology of pregnancy and childbirth
Chapter 5.2 described the roles of the reproductive system of both the male and female, including the production of gametes, and the provision of an environment in which these gametes can meet to produce a zygote. This section explores what happens following the time of conception, which is when the gametes meet, and follows the stages of development of the embryo and fetus, together with the physiological changes that occur in the mother at this time. The section concludes with a description of the physiology of childbirth and of the puerperium, which is the six-week period that follows childbirth, during which time the mother’s body begins to return to its pre-pregnant state.
The formation of the embryo
Conception is the term used to describe the fusion of one of the millions of spermatozoa in the ejaculated sperm with the ovum, so forming a fertilized egg (zygote). In natural conception this fusion most commonly occurs in one of the fallopian tubes, within a few hours of ovulation. It is estimated that the ovum is viable for no more than 48 hours. This provides the spermatozoa with a relatively short window of opportunity to meet up with the ovum in any one menstrual cycle.
A spermatozoon reaches the fallopian tube from the vagina by means of the waving action of its flagellum (the tail-like process that projects from its cell membrane). The energy required for this feat of long-distance swimming is derived from nutrients contained in the seminal fluid.
The exact timing of the fertile period is unpredictable, as it depends on the timing of ovulation, which is variable even for women with regular menstrual cycles. It is recognized that women with regular cycles can ovulate earlier than day 7 and later than day 21 in a 28-day cycle. This, of course, has implications for the natural family planning methods that can be used either to plan conception or to prevent pregnancy. This is why recognition of the changes in the consistency of cervical mucus is so important for the success of these methods.
Out of the millions of spermatozoa that are ejaculated, only a few hundred reach the fallopian tubes. When these spermatozoa reach the ovum, only one will be permitted to penetrate the cell membrane of the ovum. As soon as penetration has occurred, there is a change in the cell membrane that prevents the remaining spermatozoa from attaching (see Figure 5.3a-I). The nuclei of the male and female gametes contain only 23 single chromosomes, rather than 23 pairs. After fusion of the ovum and sperm, their nuclei fuse to form 23 pairs of chromosomes, giving a combination of genes that is totally unique to that zygote. Within a few hours this new nucleus divides, and the zygote begins to form into the ball of cells known as the embryo.
Spermatozoa that did not manage to penetrate the egg can be seen outside the cell membrane.
The tiny ball of cells of the embryo is propelled down the fallopian tube, and thence into the uterine cavity by means of the wave-like action of the ciliary epithelium of the fallopian tube. As the embryo continues to divide, a fluid-filled cavity forms in its middle. By about seven days after fertilization, the outer layer of the developing embryo has differentiated into cells that can penetrate deeply into the endometrial lining. It is these cells that will form the placenta. These outer cells have proteins on their surface that can recognize and attach to the endometrial lining. This attachment occurs seven to ten days after conception. After attachment, these outermost embryonic cells release a hormone known as human chorionic gonadotropin (HCG) into the bloodstream of the mother. This hormone signals to the corpus luteum in the ovary that pregnancy has occurred, and thus triggers the chain of physiological effects that are indicators to the mother of the start of pregnancy.
It is speculated that the reason for infertility in those couples who are shown by investigations to have no obvious physical abnormality is due to a failure in implantation. In these couples, despite healthy-looking sperm and normal ovulation, pregnancies do not occur. Subtle defects in the formation of the corpus luteum may lead to a poorly developed endometrium in the second half of the menstrual cycle. A poorly developed endometrium may either prevent implantation from occurring at all, or may be the cause of a very early miscarriage. This problem can result from an imbalance in the production of the pituitary hormones, follicle-stimulating hormone (FSH) and luteinizing hormone (LH), which may, in turn, be influenced by variables such as emotional stress, poor diet and tiredness.
Development of the embryo after implantation
After implantation, the identical cells forming this initially tiny hollow ball continue to multiply and differentiate. Differentiation is the term given to the process whereby cells with an identical set of chromosomes become specialized to form all the different tissues of the body. At implantation, the outer portion of this ball of cells forms into the embryonic part of the placenta. Within the ensuing 14 weeks, tiny blood vessels form within this tissue. This fleshy mass encourages a reciprocal development of the endometrial lining into which it penetrates. A very rich blood supply forms as this tissue develops, in which maternal and embryonic blood vessels run very close to one another. This permits an exchange of oxygen and nutrients from the mother’s blood to that of the embryo, but without any mixing of the two blood supplies. As well as oxygen and nutrients, drugs and toxins are able to cross this boundary between the mother and the embryo. It is this combination of maternal and embryonic tissue that is destined to form the placenta. The placenta will be the source of nourishment for the baby until the time of delivery.
Another portion of the hollow ball of embryonic cells is destined to become the embryo itself. This portion develops within the fluid-filled cavity. By two weeks there is a definite lengthening of this mass of cells, so forming the primitive spinal cord. By three weeks a primitive heart has formed, with vessels connecting to those of the placenta. By four weeks a tube has formed from top to bottom, which will later develop into the gastrointestinal system, and by six weeks the first features of the kidneys and sex organs have appeared. By seven weeks all the important organs have appeared. By this time there is a distinct form to the head, with bumps for the nose and ears, and also to the limbs, with tiny buds for the fingers and toes (see Figure 5.3a-II).
Figure 5.3a-II The development of the embryo at (a) six weeks and (b) nine weeks of gestation
The sac of fluid within which the embryo develops is called the amniotic sac, and the fluid within this sac is called amniotic fluid. The embryo is suspended in the fluid by a bridge of tissue known as the umbilical cord. The blood vessels that link the embryo to the placental blood supply pass through the umbilical cord. The amniotic fluid serves as a protective space for the fetus, which, until the very last weeks of pregnancy, can permit unrestricted movement of the developing limbs. This fluid serves to keep the skin moist and nourished, and is also a space into which the first urine is excreted.
The stage of pregnancy (gestation) is dated in a different way to the actual age of the embryo from the time of conception. Gestation is the term used to describe the time elapsed since the date of the last menstrual period. This is, of course, on average two weeks longer than the actual time after conception. For example, in a woman who is eight weeks pregnant, the embryo is in fact only six weeks old.
Many women will be unaware of the possibility of pregnancy for the first eight weeks of gestation, and this is particularly the case for those women who have a light bleed at four weeks, despite the fact that they are pregnant. The embryo is particularly vulnerable to nutritional deficiencies, drugs and toxins in this short time. Therefore, the only way by which a woman can ensure optimum health for her developing baby is to start making lifestyle changes before she falls pregnant. This is the foundation of the practice of giving pre-conceptual advice to couples who are planning to have a baby. The importance of giving pre-conceptual advice is emphasized in both conventional and alternative practice.
From a conventional medical perspective, a couple planning to conceive should be aware that all drugs and toxins to which the mother is exposed might also affect the developing embryo. The prospective parents will be advised to stop smoking and to avoid alcohol and other recreational drugs. A healthy diet is advised. Ideally, if possible, all therapeutic drugs should be avoided for a few weeks before conception. Women who have been taking the contraceptive pill might be advised to stop this form of contraception for at least three months before trying to conceive. The only nutritional supplement that is routinely advised are the vitamins folic acid and low-dose vitamin D. Deficiency of dietary folic acid is strongly linked with an increased incidence of spinal cord defects, including spina bifida. The advice is to take a daily supplement of this vitamin from the time of trying to conceive to at least 20 weeks of gestation. Alternative practitioners may advise a wide range of other nutritional supplements, including zinc, manganese and selenium.
Development of the fetus to the time of delivery
From about ten weeks of gestation the embryo is termed a fetus. The fetus has primitive limbs and facial features, and all the rudimentary organs in place. At this time the fetus is about 2.5cm in length.
Nourishment of the fetus
The fetus obtains the nutrients it requires from the mother’s circulation. It is generally believed in conventional medicine that, except in cases of extreme starvation, the fetus can obtain all it needs from the mother’s bloodstream, and so will not necessarily suffer if the mother is unable to have a good diet, due, for example, to morning sickness. Deficiency of folic acid in the first trimester, and of vitamin D throughout pregnancy, are exceptions to this, as these are routinely recommended as supplements in pregnancy. This is in contrast to the perspective of Chinese medicine, in which the precise content of the diet of the pregnant woman is believed to have a very important bearing on the health of the developing organs of the fetus.
However, the mother may suffer as her nutrients are being diverted to nourish the developing fetus and support the growth of the placenta. Deficiencies of iron and calcium are well recognized to affect women in pregnancy. These can than lead to anemia and weak teeth. It is very likely that many more vitamin and mineral deficiencies can occur, which may partly explain the specific food cravings that many women experience during pregnancy. Although some of the foods craved may be bizarre in nature, the most common cravings are for fruit juice and dairy products, indicating a need to replace vitamins such as vitamin C and minerals such as calcium.
The main source of energy for the fetus is glucose. Glucose is also used to make the fats required by the fetus. Excessive glucose, a problem that occurs in maternal diabetes, will stimulate growth and cause excessive fat formation, and so can lead to an overlarge baby. Maternal amino acids are used to make the proteins required to build the structure of the tissues of the fetus. The wastes that the fetus produces are removed by the placenta into the maternal blood.
The fetal circulation
The circulation of the fetus has some important differences to that of a newborn baby. The blood enters the fetus through the umbilical cord, and first passes through the liver. It is then pumped through the heart, and thence to the rest of the body, before being redirected out through the umbilical cord. One major difference in this circulatory pattern is that relatively little blood is passed through the lungs, which in the fetus are filled with fluid, and are not a source of oxygen. To enable this circulation pattern to occur, the fetal heart retains a hole in its central wall, which permits the exchange of blood between the right and left sides of the heart.
At birth, the lungs are expanded suddenly with the baby’s first cry. This expansion diverts a lot of blood into the vessels of the lungs for the first time. This has the remarkable effect of altering the pressure of blood within the rest of the fetal circulation so that the hole in the heart is closed by means of a flap of tissue, and the blood flow in the umbilicus diminishes markedly. Within a matter of seconds the whole pattern of blood flow in the fetus is transformed. Within a few minutes the pulsation of blood in the umbilical cord stops, and the cord can be tied off. What used to be the life source for the fetus now becomes a redundant stump, commonly known as the “belly button.”
The fetal digestive system
The gastrointestinal tract is relatively quiet in the fetus. As the fetus matures, it swallows amniotic fluid, which is then absorbed through the intestines and then excreted back into the amniotic fluid cavity by the kidneys. By the end of pregnancy the intestines of the fetus contain a black sticky form of feces called meconium. This is composed largely of epithelial cells that have been shed from the growing intestinal lining.
The fetal lungs
The fluid in the developing fetal lungs is in direct communication with the amniotic fluid through the mouth and nose. A circulation of this fluid is maintained by the breathing movements that occur with increasing regularity in the developing fetus. As the fetus reaches full maturity, the lungs secrete an oily substance, called surfactant, into the alveoli. After birth, surfactant enables the newly expanded air spaces to remain in their patent state. In premature babies, insufficient surfactant can result in a tendency for the alveoli to collapse. Deficiency of surfactant is one of the causes of the severe respiratory problems that can affect premature babies.
The fetal immune system
The immune system of the fetus begins to develop by week 20 of pregnancy. After this time the fetus is able to produce its own antibodies. Nevertheless, this response is not fully mature until some weeks after birth. The fetus gains extra protection from infection in the womb by maternal antibodies, which are able to cross the placenta. These antibodies have a life of a few weeks, and so continue to protect the newborn for a couple of months or so after the birth.
The progression of all the changes that occur in the fetus during its development are summarized in Table 5.3a-I.
Table 5.3a-I The stages in the development of the fetus
Age of gestation
Length of fetus
Stage of development of fetus
All organs present in rudimentary form
Limbs, muscles and bones all formed; baby has free movement
Sex organs developed
Heartbeat strong and fast
Hair, eyebrows, fingernails and fingerprint all formed; eyelids remain closed
Baby’s movement can be felt by the mother
Movement is vigorous; baby can respond to touch and sound
Baby can swallow, may get hiccups
Urine is formed by the kidneys
Eyelids open from time to time
At 28 weeks the baby has a good chance of survival if born prematurely
Fat is laid down beneath the skin of the baby, making it plump and less wrinkly
At 34 weeks most babies find a head-down position, and movements become more restricted due to lack of space
The physiological changes of pregnancy
The hormones of pregnancy
The developing embryo secretes the hormone HCG into the endometrium very soon after implantation. This means that at about day 21 after the last menstrual cycle the ovaries receive a signal that implantation has occurred. HCG stimulates the corpus luteum to continue to produce estrogen and progesterone. Without this stimulus, the corpus luteum would degenerate rapidly, and the resulting drop in estrogen and progesterone would lead to menstruation within a few days.
HCG is produced in large amounts during the first third (trimester) of the pregnancy. It is this hormone that can be detected from a sample of the mother’s urine and is used as the means of testing for pregnancy. A simple HCG test, which can be purchased at a pharmacy, can indicate pregnancy even earlier than week six of gestation (two weeks after the missed period).
Estrogen is secreted in increasing amounts throughout pregnancy. This is initially produced by the corpus luteum, but is later produced by the placenta. This hormone brings about the increased fleshiness that is characteristic of pregnancy. The cervix becomes swollen and soft, and the breasts and nipples enlarge. Estrogen also stimulates the production of increased vaginal discharge that is a normal feature of pregnancy. It also promotes an increased blood supply to all tissues. The amount of blood pumped through the heart increases, and the blood vessels dilate. In healthy pregnancies this results in a drop in blood pressure. The healthy glow commonly seen in middle to late pregnancy is a result of the effect of estrogen on the blood supply to the skin.
Like estrogen, progesterone is also produced initially by the corpus luteum, and then by the placenta. One important action of progesterone is to cause muscle and ligamentous relaxation. This effect is very marked on the muscular wall of the uterus, but excessive relaxation of the smooth muscle of the esophageal sphincter, the bowel and the ureters may also occur. This effect of progesterone can result in some of the most common minor complications of pregnancy, including heartburn, constipation and tendency to urinary infections. Relaxation of the ligaments of the pelvis and lower back is important to allow for the passage of the baby during labor. However, this can lead to some of the painful musculoskeletal conditions of pregnancy, such as low backache, sacroiliac strain and pain in the symphysis pubis. Progesterone also tends to increase the body temperature. The placidity of pregnancy is attributed to a calming effect of progesterone on the brain.
Other hormones important in pregnancy include prolactin (PRL) from the pituitary gland, adrenal corticosteroids and thyroid hormones. PRL stimulates the development of the milk-producing glands within the breast in the last ten weeks of pregnancy. The adrenal corticosteroids are important to help the body deal with the demands of pregnancy, but in excess may lead to weight gain, high blood sugar and high blood pressure. The thyroid hormones play a role in the regulation of increased growth. Under the influence of estrogen, the thyroid gland may enlarge to twice its normal size. Occasionally, if this enlargement is excessive, a goiter may develop.
The first symptoms of pregnancy
The first symptoms of pregnancy may be felt as early as the first two weeks after conception. Some women are aware of a different feeling even before the first missed period, and some may experience nausea or breast tingling at this early stage. It is more usual that symptoms are experienced at around week six of gestation. The most common include breast tingling or tenderness, nausea, and sensitivity to certain smells or tastes. The missed period is not a universal experience. Some women may have a painless bleed at four weeks, and even at eight weeks of gestation, despite having a healthy pregnancy. As well as being a potential source of great anxiety, this form of bleeding can very much confuse the dating of the pregnancy, as the estimate of the length of the gestation normally relies on the date of the last menstrual period.
The effects of the enlarging uterus
As the embryo develops into a rapidly growing fetus, the uterus and the placenta enlarge to accommodate the growth and increasing nutritional requirements. At about 12 weeks of gestation, the amniotic sac is 10cm in diameter and the uterus can be felt just above the pubic bone. At 20 weeks of gestation, the top of the uterus is at the level of the umbilicus, and by 36 weeks the uterus has reached the level of the xiphisternum. Amazingly, the remaining abdominal contents continue to function relatively normally, despite their increasingly cramped conditions.
However, certain problems can arise from the compression of the abdominal organs. In particular, compression of the abdominal aorta can lead to discomfort and faintness when lying supine (on the back) in late pregnancy. Restriction of the free flow of blood in the deep pelvic veins can contribute to the development of varicose veins, hemorrhoids and ankle swelling. Heartburn can be very severe in late pregnancy, as the uterus presses on the stomach from below. Occasionally, nerve compression can cause areas of pain and tingling. The most common site for this problem is down the front of the thigh, a result of the compression of a nerve that runs down from the crease of the groin. Rapid growth of the uterus can also lead to stretch marks and itching of the skin of the abdomen. Both these problems are compounded by the excessive weight gain that can complicate some pregnancies.
Changes in the uterus, cervix and vagina
The uterine muscle fibers lengthen markedly during pregnancy, thus allowing for the elongation of the uterus. The natural tendency for these fibers to contract becomes more pronounced. Mild contractions of the uterus start to occur from very early in pregnancy. Some, but not all, women become aware of these contractions, known as Braxton Hicks contractions from about 20 weeks of pregnancy. The sensation is usually described as a painless tightening of the abdomen, which lasts for a few seconds. Some women do describe the sensation as painful, akin to period cramps, and for this reason Braxton Hicks contractions can sometimes be confused with the onset of labor. Braxton Hicks contractions increase in frequency and intensity as the time for the onset of labor approaches.
During pregnancy, the cervix and vagina respond to the increased levels of estrogen and progesterone. The cervix becomes softer in consistency, a change that can be detected by a skilled examiner at as early as eight weeks of gestation. This change occurs in preparation for the dilatation that has to occur during the first stage of labor. The vaginal walls also change in consistency to allow for the expansion that will be necessary to allow the birth of the baby. In addition, an odorless vaginal discharge is produced. This normal feature of pregnancy can be distressing for those women who are unprepared for its occurrence.
Changes in the maternal circulation
As pregnancy progresses, the blood vessels of the mother dilate, and an increased volume of blood circulates through them. These changes are to accommodate the increased requirements of the blood vessels in the placenta. Although the actual number of red blood cells is increased in a pregnant woman, the concentration of hemoglobin within them is usually slightly less than in the non-pregnant state. This can mean that a blood test will indicate anemia. If this finding is mild, it need not be treated. However, many women will be offered treatment with iron tablets if the anemia is severe, or if symptoms such as tiredness or dizziness develop.
The heart works much harder in pregnancy, as it has to pump up to 50 percent more blood than usual. The pulse is recognized to become more full and rapid in pregnancy, and this is accompanied by feelings of warmth. The slippery quality that is a characteristic of the radial pulse in pregnancy to a Chinese medicine practitioner is possibly equivalent to this conventionally recognized pulse change.
Many women become more aware of palpitations in pregnancy. Conventionally, this symptom is not usually considered to be serious.
Changes in the respiratory, urinary and digestive systems
After about week 30 of pregnancy, the downward movement of the diaphragm becomes more restricted, and this is compensated for by an increased expansion of the ribcage during breathing. Some women will easily become breathless on exertion as this change develops.
The kidneys are more active in pregnancy, as they have to deal with the extra waste products coming from the fetus. More urine is produced. The muscles of the urinary system become more relaxed under the influence of progesterone. As previously mentioned, this can lead to an increased tendency to bladder infections. The risk of these infections spreading to involve the kidneys (pyelonephritis) is also increased in pregnancy.
The most common changes to affect the digestive system are spongy gums, heartburn and constipation. All these changes are results of the effect of progesterone. There is also an increased tendency to gum disease (gingivitis). For this reason, women are encouraged to make at least one visit to the dentist during pregnancy.
By the end of pregnancy, the fetus and the placenta together should weigh just over 4 kilograms. It is also normal for the mother to put on some weight as uterine muscle and breast tissue develop and also as fat stores are laid down around the abdomen, buttocks and thighs. These fat stores will be drawn on in the first few months after birth as the mother breastfeeds. Adding up these different elements, the ideal weight gain in pregnancy is estimated to be around 11 kilograms.
However, there can be enormous variations around this ideal. Sometimes this can be because the woman herself either loses or gains weight during the pregnancy. Less commonly, abnormal weight gain may be an indication of poor fetal or placental growth, or conversely, excessive accumulation of amniotic fluid. The most common problem with weight gain in pregnancy is that too much fat is laid down. The current advice is that a pregnant woman should eat a normal nutritious diet, and should not “eat for two.”
Psychological changes in pregnancy
Ideally, the psychological changes of pregnancy are positive ones. Pregnant women often report feeling calm and happy in the later stages of pregnancy, an experience attributed to the effects of progesterone. However, 5–10 percent of women develop depression during their pregnancy. This seems to be more common in first pregnancies, and in women who have poor social support. Often in such cases there may be deep unaddressed fears about the labor, giving birth to a malformed child, or coping with caring for a baby. Also, many women grieve for the loss of their figure and freedom. Women who suffer from depression in pregnancy seem to be more likely to develop postnatal depression.
Pregnancy: comments from a Chinese medicine perspective39
According to Chinese medicine, the Blood and Yin associated with menstruation accumulate after conception in the Chong and Ren Channels in order to nourish the developing fetus. As a consequence, the mother becomes relatively Blood and Yin Deficient, and so her Yang becomes relatively Hyperactive. Liver Yang Rising or Liver Wind Rising may develop (which can manifest as migraine headaches or pre-eclampsia in later pregnancy). Also Heat can easily accumulate during pregnancy (and manifest as rashes, palpitations and indigestion).
Depletion of energy in the lower aspect of the Chong Channel in the first trimester to nourish a healthy embryo is attributed to be the cause of the Rebellious Qi of nausea of pregnancy. Stagnation of Qi in the Chong Channel as the baby enlarges later in pregnancy may lead to breathlessness and fullness in the chest.
However, in some cases the stopping of the menses may actually mean the pregnant woman has more Blood energy available to her, and so she may feel more energetic and well, particularly in the middle trimester.
The Spleen and the Kidneys are particularly important in supporting the nourishment and growth of the fetus, and so there may be a tendency in the mother to developing Deficiency of Spleen and Stomach Qi, with Accumulation of Damp and Phlegm, and also Deficiency of Kidney Qi. The Liver is responsible for the smooth flow of Qi. As pregnancy is a time of accumulation of the Vital Substances in the lower Jiao, Liver Qi can easily stagnate as the normal flow of Qi in the middle Jiao is impaired.
All these syndromes are more likely in those women who have a pre-existing imbalance in one or more of these Organ functions. Most of the complications of pregnancy can be interpreted from a Chinese medicine perspective as consequences of these common underlying syndromes.
The physiology of childbirth
All the physiological changes of pregnancy so far described are gradual in development. In a healthy pregnancy, most women adjust well to these changes as they progress over the 40 weeks of gestation. The onset of labor is quite different. All of a sudden, the woman’s body is thrown into a series of profound changes, which escalate rapidly and culminate in the birth of the baby and the placenta, usually within a timescale of 24 hours.
The initiation of labor
The precise trigger for the onset of labor is not clearly understood. It is known that the uterus becomes more sensitive to the pituitary hormone oxytocin during late pregnancy. It is likely that a surge in the release of oxytocin from the pituitary actually brings about the relentless wave of contractions that are characteristic of established labor. It may be that it is a hormonal factor released from the maturing fetus, such as an adrenal corticosteroid, that actually stimulates this surge. It is recognized that external factors, such as shock, injury and fever, can stimulate early labor, and anxiety and exhaustion can delay the progression of labor.
As the time for labor approaches, the Braxton Hicks contractions of the uterus become more frequent. Usually, at about two weeks before the onset of labor, the head of the baby drops into the bowl of the pelvis. This descent, known as engagement, often brings about a feeling of more comfort in the diaphragmatic area, as more space is created. In some pregnancies, engagement does not occur until after the onset of labor. In a few of these, this may be because the baby is not in the correct head-down, posterior-facing position.
Established labor is characterized by the onset of regular painful contractions that cause the tiny canal of the cervix to start to widen (dilate). In many women this transition between Braxton Hicks contractions and labor contractions is not clear-cut. In these women there may be a period of a few days in which the contractions are mildly painful and regular from time to time. Vaginal examination during this time will show that the cervix has not yet started to dilate. As cervical dilatation begins, a plug of mucus is released, which appears as an odorless blood-stained discharge sometimes called a “show.”
Healthy established labor involves regular contractions, which gradually become more frequent and sustained. A typical pattern is that the contractions start to occur at intervals of over 15 minutes and last for only a few seconds. As the cervix gradually dilates, the interval between contractions shortens to less than 5 minutes, and the pain of the contraction lasts for 1–2 minutes. As this progression occurs, the contractions become more painful. Initially, the woman will be able to carry on usual activities between the contractions. The pain at this stage is often compared to that of period cramps. It is usually felt in the lower abdomen and low back.
In some women, the amniotic sac ruptures at this early stage, leading to a trickle or a gush of clear fluid. Occasionally, this rupture occurs before the onset of labor. This can then stimulate the onset of labor. If labor does not start within 24 hours of the breaking of the waters, there is a potential risk of infection, and so labor may have to be induced medically.
Labor is described in terms of three stages. The show and the onset of regular painful contractions herald the first stage of labor. This is the part of labor in which the canal of the cervix dilates to a diameter of about 10cm. In this stage there is only a small downward movement of the baby.
In first pregnancies, a normal first stage can last up to 36 hours, although less than 24 hours is more usual. For many women who have a prolonged first stage, for much of this period the initial dilatation of the cervix is slow and the contractions are bearable. Usually, it is only in the later part of the first stage that the contractions become very strong. In subsequent pregnancies the first stage lasts only a few hours, and the progression to very strong contractions is more rapid.
As the first stage of labor becomes established, the pain of the contractions increases markedly. As the cervix dilates to greater than 5cm, the pain causes the woman to go inward, stop talking and lose eye contact with the birth attendants. The respite between the contractions shortens down to only a few seconds as the contractions last for longer and become more frequent. If this painful stage is prolonged, the woman can become exhausted. For this reason, it is now common practice to encourage the woman to continue to eat light starchy food in the early part of the first stage of labor. Midwives often recommend glucose drinks in the later part of the first stage, to sustain the woman as she goes through this marathon.
The transition between the first and second stages of labor
The transition time is when the cervix reaches its maximal dilatation. It is at this time that contractions are at their most painful. Very often, the woman feels totally overwhelmed by what she is experiencing. Some women experience great fear or panic, believing that they are about to die. Other women become aggressive and abusive. Some may insist that they can no longer cope with the pain, and might demand medical pain relief. However, these behaviors are often signs that labor is progressing well. This transitional time usually lasts less than 30 minutes.
Occasionally, the woman has a desire to push during the transitional stage. If this occurs, she may be advised by the midwife to avoid pushing, until it is certain that the cervix is fully dilated. This is because a premature onset of pushing can cause the cervix to tear.
It is usually around the time of transition that the waters break if they have not done so already.
The second stage of labor
The second stage of labor is the stage in which contractions of the uterus are combined with the woman voluntarily pushing her abdominal muscles. This combined action forces the baby downwards through the pelvic canal. The second stage ends with the birth of the baby. It should last no more than 2 hours.
The second stage is characterized by the experience of the urge to push. Generally, the contractions are very frequent and strong, but may be more bearable than in the first stage. This means that if a woman can be supported through the transition without resort to medical pain relief, she may not need to request it for the second stage.
The forceful downward contractions encourage the head of the baby to descend down the pelvic canal, usually with the face directed backwards. The mother begins to feel an intense pressure as the head enters the vagina. Very commonly, women panic at this stage, fearing that they are going to open their bowels. This is because the feeling of fullness can also be sensed in the rectum. The downwards descent of the head can be seen by the birth attendant a few contractions before the baby is actually delivered.
Once the head has reached the perineum, the surrounding tissues become very tightly stretched. At this point the woman feels an excruciating bursting sensation. It is at this stage that the tissues of the vaginal wall and perineum may tear. Ideally, tearing can be prevented by controlling the force of the contractions. To enable this, the midwife will usually advise the women when to push and when to avoid pushing, by the use of breathing exercises. The midwife will also try to control the descent of the baby’s head with manual pressure. However, the instinctive urge to push is often too great to be controlled, and tearing cannot be avoided. The practice of episiotomy, in which the midwife makes a cut in the perineum, is now becoming outmoded. It is recognized that most natural tears heal more efficiently than the surgical cut of the episiotomy.
Once the head has been delivered, the midwife checks that the umbilical cord is not tightly wrapped around the baby’s neck. If it is, the midwife will free the cord before the rest of the baby’s body is delivered. This is a common occurrence, which, if managed correctly, does not usually have serious consequences. Delivery of the rest of the baby’s body usually occurs within another one or two contractions. A healthy newborn baby will make a cry within a few seconds. If there are no complications, it is common practice to hand the baby to the mother straight away to allow it to suckle. Ideally, time should be given (2–3 minutes) to allow the pulsation of blood in the vessels of the umbilical cord to stop before the cord is clamped and cut.
Once the baby has been delivered, there is usually a dramatic cessation of pain and discomfort for the mother. She often experiences a surge of renewed energy as she first sees and holds her new baby.
The third stage of labor
The third stage of labor involves the delivery of the placenta and the amniotic membranes (afterbirth). By the time of the labor, the placenta is a grapefruit-sized mass of soft tissue, with the color and consistency of fresh liver. After the umbilical vessels have stopped pulsating, the placental blood vessels contract, so allowing the placenta to come away from the wall of the uterus without excessive blood loss. After about 10–20 minutes, continued contractions of the uterus force the separated placenta, amniotic membranes and remaining umbilical cord out through the vagina. A smooth third stage is enabled by the midwife, who applies pressure to the uterus, and controlled traction to the cord as the placenta passes out of the uterus.
The uterus continues to contract after the expulsion of the placenta, so that its rich network of blood vessels is encouraged to shut down. These continued contractions are encouraged by the suckling of the baby, as this stimulates a natural release of oxytocin from the mother’s pituitary gland.
To facilitate this process, it is very common practice to administer an injection of an oxytocin-like drug called syntometrine to the mother. This is intended to speed the contraction of the uterus so that blood loss is minimized. This injection is usually given into the mother’s thigh just after the delivery of the baby’s head. This treatment is believed to reduce the incidence of severe blood loss by half. However, if it is given, the umbilical cord needs to be clamped as soon as the baby is delivered to prevent the drug reaching the baby. Syntometrine may cause headaches and nausea in the mother. The significant complication of syntometrine use is that the uterus may contract prematurely around the placenta, so preventing its removal. If this occurs, a minor operation is required so that the placenta can be removed manually.
The contractions of the third stage are much less painful than those of the first and second stages. Often the mother may be unaware that this stage has taken place, as she is so involved with holding and suckling her newborn baby.
After the delivery of the placenta, the midwife inspects it to check that it is intact, and to estimate the approximate blood loss. The amount of blood lost in an average labor is around 400 milliliters. An amount greater than 500 milliliters is considered excessive, and replacement therapy, either by means of iron or blood transfusion, may be considered necessary. If there has been a tear, this is then stitched, either by the midwife or the doctor in attendance.
The effect of labor on the fetus
During labor, the head of the human fetus experiences much greater pressure than occurs during the birth of other mammals. The soft, immature plates of bone that form the skull are often molded into an elongated shape during the passage of the head through the pelvic canal. A more normal shape is reassumed over the course of the few days after birth. Severe molding can occur if the second stage is prolonged, or if assisted methods of delivery such as forceps have to be used.
The baby always suffers from a period of low oxygen supply during labor. This can affect the heart rate of the baby, which is monitored by the midwife. A prolonged period of abnormal fetal heart rate is taken seriously. If this occurs, it may lead to the use of medical interventions to speed up the labor, or the decision to proceed to a cesarean section.
Another response of fetal distress is that the fetus opens its bowels during the course of labor. The appearance of black, sticky meconium in the leaking amniotic fluid is a warning sign of fetal distress in labor. If meconium has been released, not only does this indicate possible distress in the fetus, but there is also a risk of the meconium being inhaled by the fetus. Therefore, the release of meconium is another reason why medical intervention may have to take over the course of the labor, so that the baby is delivered as soon as possible.
There is no doubt that even uncomplicated labor is stressful for the fetus. The fetal adrenal glands are stimulated to produce excessive amounts of adrenaline and corticosteroids in response to this stress. It is believed that high levels of these hormones are important in promoting the dramatic changes that occur in the cardiovascular and respiratory systems of the baby just after birth.
Information box 5.3a-II
Labor: comments from a Chinese medicine perspective
In Chinese medicine, labor is believed to be a time when Yang expels Yin. After 40 weeks of relative Stagnation of Qi in the Lower Jiao, a powerful and smooth downward movement of Qi leads to the expulsion of the fetus. This has also been described in terms of Qi moving Blood. A healthy labor is dependent on healthy Blood and Qi. Healthy Spleen, Kidney and Liver Organs are particularly important in ensuring that a labor progresses smoothly through all three stages.