What are birth defects?
As the human embryo develops, it undergoes many formative stages from the simple to the complex, most often culminating in a perfectly formed newborn infant. The formation of the embryo is controlled by genetic factors, external influences, and interactions between the various embryonic tissues. Because genes play a vital role as the blueprint for the developing embryo, they must be unaltered and the cellular mechanisms that allow the genes to be expressed must also work correctly. In addition, the chemical and physical communications between cells and tissues in the embryo must be clear and uninterrupted. The development of the human embryo into a newborn infant is infinitely more complex than the design and assembly of the most powerful supercomputer or the largest skyscraper. Because of this complexity and the fact that development progresses without supervision by human eye or hand, there are many opportunities for errors that can lead to malformations.
Errors in development can be caused by both genetic and environmental factors. Genetic factors include chromosomal abnormalities and gene mutations. Both can be inherited from the parents or can occur spontaneously during gamete formation, fertilization, and embryonic development. Environmental factors, called teratogens, include such things as drugs, disease organisms, and radiation.
Chromosomal abnormalities account for about 6 percent of human congenital malformations. They fall into two categories, numerical and structural. Numerical chromosomal abnormalities are most often the result of nondisjunction occurring in the germ cells that form sperm and eggs. During the cell division process in sperm and egg production, deoxyribonucleic acid (DNA) is duplicated so that each new cell receives a complete set of chromosomes. Occasionally, two chromosomes fail to separate (nondisjunction), such that one of the new cells receives two copies of that chromosome and the other cell none. Both of the resulting gametes (either sperm or eggs) will have an abnormal number of chromosomes. When a gamete with an abnormal number of chromosomes unites with a normal gamete, the result is an individual with an abnormal chromosome number. The missing or extra chromosome will cause confusion in the developmental process and result in certain structural and functional abnormalities. For example, persons with an extra copy of chromosome number 21 suffer from Down syndrome, which often includes mental deficiency, heart defects, facial deformities, and other symptoms and can be caused by nondisjunction in one or more cells of the early embryo. Abnormal chromosome numbers may also result from an egg’s being fertilized by two sperm, or from failure of cell division during gamete formation.
Structural chromosomal abnormalities result from chromosome breaks. Breaks occur in chromosomes during normal exchanges in material between chromosomes (crossing over). They also may occur accidentally at weak points on the chromosomes, called fragile sites, and can be induced by chemicals and radiation. Translocations occur when a broken-off piece of chromosome attaches to another chromosome. For example, an individual who has the two usual copies of chromosome 21 and, as the result of a translocation, carries another partial or complete copy of 21 riding piggyback on another chromosome will have the symptoms of Down syndrome. Deletions occur when a chromosome break causes the loss of part of a chromosome. The cri du chat syndrome is caused by the loss of a portion of chromosome number 5. Infants affected by this disorder have a catlike cry, are intellectually and developmentally disabled, and have cardiovascular defects. Other structural chromosomal abnormalities include inversions (in which segments of chromosomes are attached in reverse order), duplications (in which portions of a chromosome are present in multiple copies), and isochromosomes (in which chromosomes separate improperly to produce the wrong configuration).
Gene mutations (defective genes) are responsible for about 8 percent of birth defects. Mutations in genes occur spontaneously because of copying errors or can be induced by environmental factors such as chemicals and radiation. The mutant genes are passed from parents to offspring; thus certain defects may be present in specific families and geographical locations. Two examples of mutation-caused defects are polydactyly (the presence of extra fingers or toes) and microcephaly (an unusually small cranium and brain). Mutations can be either dominant or recessive. If one of the parents possesses a dominant mutation, there will be a 50 percent chance of this mutant gene being transmitted to the offspring. Brachydactyly, or abnormal shortening of the fingers, is a dominantly inherited trait. Normally, the parent with the dominant gene also has the disorder. Recessive mutations can remain hidden or unexpressed in both parents. When each parent possesses a single recessive gene, there is a 25 percent chance that any given pregnancy will result in a child with a defect. Examples of recessive defects are the metabolic disorders sickle cell disease and hemophilia.
Environmental factors called teratogens are responsible for about 7 percent of congenital malformations. Human embryos are most sensitive to the effects of teratogens during the period when most organs are forming (organogenetic period), that is, from about fifteen to sixty days after fertilization. Teratogens may interfere with development in a number of ways, usually by killing embryonic cells or interrupting their normal function. Cell movement, communication, recognition, differentiation, division, and adhesion are critical to development and can be easily disturbed by teratogens. Teratogens can also cause mutations and chromosomal abnormalities in embryonic cells. Even if the disturbance is only weak and transitory, it can have serious effects because the critical period for the development of certain structures is very short and well defined. For example, the critical period for arm development is from twenty-four to forty-four days after fertilization. A chemical that interferes with limb development, such as the drug thalidomide, if taken during this period, may cause missing arm parts, shortened arms, or complete absence of arms. Many drugs and chemicals have been identified as teratogenic, including alcohol, aspirin, and certain antibiotics.
Other environmental factors that can cause congenital malformations include infectious organisms, radiation, and mechanical pressures exerted on the fetus within the uterus. Certain infectious agents or their products can pass from the mother through the placenta into the embryo. Infection of the embryo causes disturbances to development similar to those caused by chemical teratogens. For example, German measles (rubella virus) causes cataracts, deafness, and heart defects if the embryo is infected early in development. Exposure to large doses of radiation—such as those released by the accident at the Chernobyl nuclear power plant in 1986 or by the atomic bombs dropped on Hiroshima and Nagasaki, Japan, during World War II—can result in death and damage to embryonic cells. There was an increase of about 10 to 15 percent in birth defects in children born to pregnant women exposed to atomic bomb radiation in Japan. Diagnostic X-rays are not known to be a cause of birth defects. Some defects such as hip dislocation may be caused by mechanical forces inside the uterus; this could happen if the amnion is damaged or the uterus is malformed, thus restricting the movement of the fetus. About 25 percent of congenital defects are caused by the interaction of genetic and environmental factors (multifactorial), and the causes of more than half (54 percent) of all defects are unknown.
Because many birth defects have well-defined genetic and environmental causes, they often can be prevented. Preventive measures need to be implemented if the risk of producing a child with a birth defect is higher than average. Genetic risk factors for such defects include the presence of a genetic defect in one of the parents, a family history of genetic defects, the existence of one or more children with defects, consanguineous (same-family) matings, and advanced maternal age. Prospective parents with one or more of these risk factors should seek genetic counseling in order to assess their potential for producing a baby with such defects. Also, parents exposed to higher-than-normal levels of drugs, alcohol, chemicals, or radiation are at risk of producing gametes that may cause defects, and pregnant women exposed to the same agents place the developing embryo at risk. Again, medical counseling should be sought by such prospective parents. Pregnant women should maintain a well-balanced diet that is about 200 calories higher than normal to provide adequate fetal nutrition. Women who become anemic during pregnancy may need an iron supplement, and the U.S. Public Health Service recommends that all women of childbearing age consume 0.4 milligram of folic acid (one of the B vitamins) per day to reduce the risk of spina bifida and other neural tube defects. Women at high risk for producing genetically defective offspring can undergo a screening technique whereby eggs taken from the ovary are screened in the laboratory to select the most normal appearing ones prior to in vitro fertilization and then returned to the uterus. Some couples may decide to use artificial insemination by donor if the prospective father is known to carry a defective gene.
The early detection of birth defects is crucial to the health of both the mother and the baby. Physicians commonly use three methods for monitoring fetal growth and development during pregnancy. The most common method is ultrasound scanning. High-frequency sound waves are directed at the uterus and then monitored for waves that bounce back from the fetus. The return waves allow a picture of the fetus to be formed on a television monitor, which can be used to detect defects and evaluate the growth of the fetus. In amniocentesis, the doctor withdraws a small amount of amniotic fluid containing fetal cells; both the fluid and the cells can be tested for evidence of congenital defects by growing the cells in tissue culture and examining their chromosomes. Amniocentesis generally cannot be performed until the sixteenth week of pregnancy. Another method of obtaining embryonic cells is called chorionic villus sampling and can be done as early as the fifth week of pregnancy. A tube is inserted into the uterus in order to retrieve a small sample of placental chorionic villus cells, identical genetically to the embryo. Again, these cells can be tested for evidence of congenital defects. The early discovery of fetal defects and other fetal-maternal irregularities allows the physician time to assess the problem and make recommendations to the parents regarding treatment. Many problems can be solved with therapy, medications, and even prenatal surgery. If severe defects are detected, the physician may recommend termination of the pregnancy.
Children born with defects often require highly specialized and intense medical treatment. For example, a child born with spina bifida may have lower-body paralysis, clubfoot, hip dislocation, and gastrointestinal and genitourinary problems in addition to the spinal column deformity. Spina bifida occurs when the embryonic neural tube and vertebral column fail to close properly in the lower back, often resulting in a protruding sac containing parts of the spinal meninges and spinal cord. The malformation and displacement of these structures result in nerve damage to the lower body, causing paralysis and the loss of some neural function in the organs of this area. Diagnostic procedures including X-rays, computed tomography (CT) scans, and urinalysis are carried out to determine the extent of the disorder. If the sac is damaged and begins to leak cerebrospinal fluid, it needs to be closed immediately to reduce the risk of meningitis. In any case, surgery is done to close the opening in the lower spine, but it is not possible to correct the damage done to the nerves. Urgent attention must also be given to the urinary system. The paralysis often causes loss of sphincter muscle control in the urinary bladder and rectum. With respect to the urinary system, this lack of control can lead to serious urinary tract infections and the loss of kidney function. Both infections and obstructions must be treated promptly to avoid serious complication. Orthopedic care needs to begin early to treat clubfoot, hip dislocation, scoliosis, muscle weakness, spasms, and other side effects of this disorder.
The medical treatment of birth defects requires a carefully orchestrated team approach involving physicians and specialists from various medical fields. When the abnormality is discovered (before birth, at birth, or after birth), the primary physician will gather as much information as possible from the family history, the medical history of the patient, a physical examination, and other diagnostic tests. This information is interpreted in consultation with other physicians in order to classify the disorder properly and to determine its possible origin and time of occurrence. This approach may lead to the discovery of other malformations, which will be classified as primary and secondary. When the physician arrives at a specific overall diagnosis, he or she will counsel the parents about the possible causes and development of the disorder, the recommended treatment and its possible outcomes, and the risk of recurrence in a subsequent pregnancy. Certain acute conditions may require immediate attention in order to save the life of the newborn.
In addition to treating the infant with the defect, the physician needs to counsel the parents in order to answer their questions. The counseling process will help them to understand and accept their child’s condition. To promote good parent-infant bonding, the parents are encouraged to maintain close contact with the infant and participate in its care. Children born with severe chronic disabilities and their families require special support. When parents are informed that their child has limiting congenital malformations, they may react negatively and express feelings of shock, grief, and guilt. Medical professionals can help the parents deal with their feelings and encourage them to develop a close and supportive relationship with their child. Physicians can provide a factual and honest appraisal of the infant’s condition and discuss treatments, possible outcomes, and the potential for the child to live a happy and fulfilling life. Parents are encouraged to learn more about their child’s disorder and to seek the guidance and help of professionals, support groups, family, and friends. With the proper care and home environment, the child can develop into an individual who is able to interact positively with family and community.
Birth defects have been recognized and recorded throughout human history. The writer of the Old Testament book of 2 Samuel (21:20) describes the defeat of a giant with six fingers and six toes. Defects were recorded in prehistoric art, and the cuneiform records of ancient Babylon considered birth defects to be omens of great significance. Aristotle described many common human birth defects such as polydactyly. Superstitions about birth defects abounded during the Middle Ages. People believed that events occurring during pregnancy could influence the form of the newborn; for example, deformed legs could be caused by contact with a cripple. Mothers of deformed children were accused of having sex with animals. In a book written about birth defects in 1573, Monstres et prodiges, Ambroise Paré describes many human anomalies and attempts to explain how they occur. Missing body parts such as fingers or toes were attributed to a low sperm count in the father, and certain characteristics such as abnormal skin pigmentation, body hair, or facial features were said to be influenced by the mother’s thoughts and visions during and after conception.
With advances in science and medicine these superstitions were swept aside. Surgery for cleft palate was performed as early as 1562 by Jacques Honlier. William Harvey, a seventeenth-century English physician, recognized that some birth defects such as cleft lip are normal embryonic features that accidentally persist until the time of birth. The study of embryology, including experiments on bird and amphibian embryos, blossomed as a science during the nineteenth century, leading to a better understanding of how defects arise. At the same time, physicians were developing improved ways to treat birth defects. By 1816, Karl von Graefe had developed the first modern comprehensive surgical method for repairing cleft palate. The modern technique for repairing congenital pyloric stenosis (narrowing of the junction between the stomach and small intestine) was developed by Conrad Ramstedt in 1912. The principles of genetic inheritance developed by Gregor Mendel in the mid-nineteenth century were rediscovered by biologists at the beginning of the twentieth century and soon were applied to the study of human heredity, including the inheritance of birth defects. Geneticists realized that defects such as hemophilia and Down syndrome are inherited diseases. Beginning in the 1930s, other scientists began to show that congenital defects could be induced in experimental animals by such factors as dietary deficiencies, hormone imbalances, chemicals, and radiation. In some cases, a lack of complete testing of environmental factors such as drugs has led to tragedies but also a better understanding of the nature of birth defects. The tranquilizer thalidomide caused limb malformations in more than seven thousand children in Europe before it was withdrawn from the market in 1961. Pregnant women treated for cervical cancer in the 1960s with large doses of radiation bore children with defects and developmental disabilities.
Indeed, much of the medical and environmental health research today centers on the effects of drugs, toxic chemicals, radiation, and other factors on human health and development. Genetic counseling and testing of parents at risk for inherited defects has become an accepted part of medical practice. In addition, there have been many advances in the treatment of congenital defects since the 1950s. Modern orthopedic and plastic surgery is used to correct such problems as clubfoot and cleft palate. Transplants are used to correct deficiencies of the liver, kidneys, and other organs. Biomedical engineers have developed improved prosthetic devices to replace lost limbs and to aid in hearing, speaking, and seeing. An understanding of metabolic disorders such as phenylketonuria (PKU) has led to better treatment that utilizes special diets and medications. Because it is difficult to undo the damage of congenital defects fully, the most promise seems to be in the areas of prevention and protection. Prospective parents and their medical care providers need to be alert to potential hereditary problems, as well as to exposure to hazardous environmental agents. Pregnant women need to maintain a healthy diet and check with their physicians before taking any drugs. With advances in preventive medicine, diagnosis, and treatment, the future is much brighter for reducing the health toll of congenital malformations.
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