Causes and Symptoms (Magill’s Medical Guide, Sixth Edition)
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...
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Treatment and Therapy (Magill’s Medical Guide, Sixth Edition)
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...
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Perspective and Prospects (Magill’s Medical Guide, Sixth Edition)
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...
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For Further Information: (Magill’s Medical Guide, Sixth Edition)
Heyman, Bob, and Mette Henriksen. Risk, Age, and Pregnancy: A Case Study of Prenatal Genetic Screening and Testing. New York: Palgrave, 2001. Examines prenatal testing in the context of a British hospital, exploring the perspectives of pregnant women, hospital doctors, and midwives, and the way in which the decision for prenatal testing is made.
March of Dimes. http://www.marchofdimes.com. Web site offers a range of excellent fact sheets on myriad birth defects, information about prenatal testing, and special sections for pregnant women and researchers and professionals.
Moore, Keith L., and T. V. N. Persaud. The Developing Human. 8th ed. Philadelphia: Saunders/Elsevier, 2008. An outstanding textbook on human embryonic development, with specific information about the causes of congenital malformations and common defects occurring in each of the body’s systems.
Nixon, Harold, and Barry O’Donnell. The Essentials of Pediatric Surgery. 4th ed. Boston: Butterworth Heinemann, 1992. Describes in accessible terms the surgical treatment of many congenital abnormalities, including birth injuries, imperforate anus, spina bifida, hydrocephalus, pyloric stenosis, birthmarks, cleft lip and palate, and hernias.
Sadler, T. W. Langman’s Medical Embryology. 11th ed. Philadelphia: Lippincott Williams & Wilkins, 2009. Text that covers the fundamentals of embryology with...
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Birth Defects (Encyclopedia of Medicine)
Birth defects are physical abnormalities that are present at birth; they are also called congenital abnormalities. More than 3,000 have been identified.
Birth defects are found in 2-3% of all newborn infants. This rate doubles in the first year, and reaches 10% by age five, as more defects become evident and can be diagnosed. Almost 20% of deaths in newborns are caused by birth defects.
Abnormalities can occur in any major organ or part of the body. Major defects are structural abnormalities that affect the way a person looks and require medical and/or surgical treatment. Minor defects are abnormalities that do not cause serious health or social problems. When multiple birth defects occur together and have a similar cause, they are called syndromes. If two or more defects tend to appear together but do not share the same cause, they are called associations.
Causes and symptoms
The specific cause of many congenital abnormalities is unknown, but several factors associated with pregnancy and delivery can increase the risk of birth defects.
Any substance that can cause abnormal development of the egg in the mother's womb is called a teratogen. In the first two...
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Birth Defects (Encyclopedia of Science)
Birth defects, or congenital defects, are abnormalities that are present at birth. They may be the result of genetic factors such as an inherited disease or a chromosomal abnormality. They may also be caused by environmental factors such as radiation, the mother's use of drugs or alcohol while pregnant, or bacterial or viral infections.
Birth defects can range from very minor, such as a birthmark, to a more serious condition that results in physical deformity or limits the lifespan of the child. Two to three percent of babies born in the United States have a major birth defect.
Some birth defects are caused by chromosomal abnormalities. Chromosomes are structures that are found in the nucleus of every human cell and that contain genes, the units of heredity. Genes are responsible for the physical traits and genetic makeup of an individual. A human fetus inherits 23 chromosomes from its mother and 23 chromosomes from its father, making a total of 46 chromosomes in 23 pairs.
The most common chromosomal abnormalities seen in humans involve an extra chromosome or a missing chromosome. Down syndrome (named for nineteenth-century English physician J. L. H. Down) is a birth disorder caused by the presence of an extra chromosome, so that a fetus with this condition has 47 chromosomes instead of 46. Down...
(The entire section is 927 words.)