Pregnancy and Childbirth

5.3a The physiology of pregnancy and childbirth

Spermatozoa that did not manage to penetrate the egg can be seen outside the cell membrane.

Implantation

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.

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.

Pre-conceptual advice

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

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.

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

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.

Weight gain

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.

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 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.