Birth defects are abnormalities of the body structure or chemistry that occur while the fetus * is developing in the mother's uterus. Hereditary or genetic causes, environmental influences, or a combination of factors may affect the embryo or fetus before it is born.
There have been a number of instances from the 1960s onward when babies were born with severe and disturbing abnormalities. In 1994 and 1995 a number of infants along the Rio Grande in Texas and Mexico were born without a brain, a condition called anencephaly. Some professionals suspected that the birth defects occurred when the mothers were exposed to pollution in the environment. There are many unknowns about birth defects; however, scientists know that the human embryo is exceedingly vulnerable to drugs, viruses, and radiation during the first several months when its most important body systems are forming
Some birth defects are inherited from parents, and others are acquired through a parent's contact with the environment or with an infectious disease. According to the Centers for Disease Control and Prevention, 3 percent of babies born in the United States have some type of birth defect. Birth defects account for more than 20 percent of infant deaths.
Some birth defects, such as anencephaly, occur infrequently. Others, such as some congenital * heart defects, are more common. Some hereditary defects are more common in certain populations than in others, and some are more common in one sex or the other.
Traditionally, geneticists were scientists who studied genes and chromosomes, and embryologists were doctors who studied the development of fetuses. The development of powerful electron microscopes has allowed scientists to combine research into genetics * and embryology * Genetics
About 6 feet (1.83 meters) of deoxyribonucleic acid (DNA) are packaged in the nucleus * of most cells into 23 pairs of chromosomes * , threadlike structures inside cells on which the genes * are located. Each gene is a segment of DNA that holds the recipe for making a specific molecule, usually a protein. These recipes are spelled out in varying sequences of the four chemical bases in DNA: adenine (A), thymine (T), guanine (G), and cytosine (C). A is always paired with T; G is always paired with C. The two sets of bases are called base pairs. The DNA molecule looks like two ladders with a side taken off and twisted together; the rungs of the ladders are the base pairs. Millions of these base pairs may be needed to make up a single gene. These genes direct the growth and characteristics of the organism through the production of proteins. Proteins are made up of amino acids and are the essential components of all organs and chemical activities. Their functions depend on their shape and are determined by the 20,000 to 25,000 genes in the cell nucleus. The complete makeup of the genes of an individual is called the genome. Of course, with so many chemical reactions going on, various problems may develop.
The genetic material or DNA that a person inherits begins in the reproductive systems of the parents. Sperm and eggs (ova), or cells called gametes, carrying half of the parent's genes are created through a process called meiosis * , or reduction division. In this complex process, the parent cells in the testes (sperm) of the male and the ovaries of the female (ovum) reduce their DNA by half so that they can join together and make a fertilized egg that contains one set of genes from the mother and one set from the father. Human body cells, or somatic cells, have the full set of 46 chromosomes. However, each gamete has only half that number, or 23. In males, meiosis takes place in the testicles and occurs about every 74 hours, beginning at about 12 to 13 years of age and continuing through life. Millions of sperm are produced daily, but as many as 20 percent may be defective.
A more complex process of meiosis occurs in females, beginning about the fifth month after conception when immature ova are formed. The ova remain in a premature form in the ovaries until hormones signal their release. The egg does not complete its development until conception, when chemical changes occur as the main part of the sperm enters the ovum. Unlike males and sperm production, females are born with all the ova they will ever have. At fertilization or conception, the 23 chromosomes of the father and the 23 chromosomes of the mother join to form a completely new individual with 46 chromosomes and a unique genetic makeup.
During the first week, the ovum that has been fertilized in the fallopian tube reaches the uterus and implants in the uterus. At about 12 days after conception, a band called the primitive streak appears along the back of the mass of cells. The neural tube, which develops into the spinal cord and brain, begins to develop and continues to develop throughout pregnancy. At about five weeks, the general form of the human body becomes evident, with traces of hands, feet, clavicle (collarbone), and lower jaw. During the first 12 weeks of development, the cells of the embryo are rapidly dividing and forming muscles, bones, and organs. Although the embryo is most vulnerable during the first three months, the fragile fetus is susceptible to many factors that affect development at any time during the remaining six months.
Hereditary birth defects are caused when a mistake is made as DNA divides and forms a template to transcribe copies. Then a process called translation enters in to create the final product. Many disorders of genetic origin occur during translation; the final product will be a mutation or change from the normal final product. One might compare the process to a group of workers on an assembly line making peanut butter and jelly sandwiches. Each person has a job to do: unwrapping the loaf of bread; spreading the peanut butter; spreading the jelly; and putting the top and bottom slices together. The process moves like clockwork, but if the jelly spreader drops the knife and fails to complete the job, there will be no jelly on that sandwich. This simple illustration shows how one small slip can change the outcome of a product. Likewise, even small changes in the primary structure of a protein may affect the protein's properties. For example, in sickle cell anemia *
Some diseases develop through random mutation to act as protection against environmental conditions in certain areas. In the 1940s Anthony Allison, a South African geneticist, studied the blood of people living in areas of Africa that had high rates of malaria. He found that those individuals who had sickle cells, and even those with only one defective gene, who therefore had the sickle cell trait and not the disease, were less likely to contract malaria. Consequently, individuals with a mutant gene for hemoglobin have a survival advantage when they live in countries where malaria is common. This correlation explains why the mutant gene is so widely distributed in those areas. However, the gene was carried to the United States during the period of slavery before the Civil War, and persists in the African American population in the early 21st century even though malaria is extremely rare in the United States. Likewise, Tay-Sachs disease evolved among the Ashkenazi Jews from a single town in Poland. The mutations offered the carriers protection against tuberculosis but had a lethal effect on some of the population. Current incidences of sickle cell anemia and Tay-Sachs disease are a result of diseases of the past. Fortunately, because of genetic testing and counseling, Tay-Sachs disease is rare in the early 21st century. Because of preimplantation genetic diagnosis, fertilized ova that lack the mutated gene for sickle cell can be selected for implantation, ensuring the birth of a child without the disease.
Genetic birth defects can be dominant or recessive single-gene traits, X-linked disorders, multigene traits, or chromosomal abnormalities.
Starting at conception, the developing embryo is susceptible to many factors that affect development. Birth defects can also be caused by environmental factors, either alone or together with faulty genes. Environmental factors include the environment in the mother's uterus, or womb, and the possible influence of toxic materials in the earth's environment.
The following are some common types of serious brain defects and their possible causes:
During the first weeks of embryonic development, exposure to toxins may disrupt the development of the fetus and cause serious birth defects. Certain agents may have a teratogenic (TER-ah-to-JEN-ik) effect on the embryo. The word teratogen literally means “giving rise to monsters.” There are several known teratogens:
Environmental hazards may act as teratogens. Exposure to chemicals or other toxins in the environment may contribute to birth defects. Chemical pollutants in water supplies, including mercury compounds, phenols, pesticides, lead, and aromatic hydrocarbons have been shown to cause birth defects, as have lead and dioxin in the soil. Pregnant women should avoid weed killers, insecticides, and even certain household cleaning agents.
Since the 1940s there has been concern about the possible role of radioactive materials and chemical agents used in warfare leading to an increased number of birth defects in populations exposed to these agents. While the effects on the health of adults of radiation from atomic bombs and nuclear accidents like the Chernobyl disaster in 1986 have been well documented, the relationship of radiation exposure to birth defects in particular is less well understood as of 2016. It is clear, however, that nuclear and chemical weaponry is as potentially dangerous to fetuses in the womb as contaminated drinking water and soil, lead paint in buildings, and air pollution.
A number of infectious diseases can also affect the developing embryo and fetus. Some of the diseases may not affect the mother but can be passed on to the unborn fetus. Infections that may have major effects on the fetus include:
Some birth defects can be diagnosed while the unborn child is still in its mother's womb. A procedure called ultrasound, which uses sound waves to produce an image of a fetus on a screen, can detect some malformations such as spina bifida.
In a procedure called amniocentesis (am-nee-o-sen-TEE-sis), a small sample of fluid surrounding the fetus is removed through a needle and examined. This test is useful in detecting inborn metabolic (body chemistry) defects and abnormalities in the chromosomes.
Many birth defects can be diagnosed by a doctor's physical examination of a newborn baby. Other tests, including x-rays, may be ordered if doctors suspect a birth defect; blood tests can detect certain disorders of the blood or body chemistry. Many infants with defects can develop normally if they receive prompt treatment.
See also Cerebral Palsy • Cleft Palate • Clubfoot • Cystic Fibrosis • Cytomegalovirus (CMV) Infection • Deafness and Hearing Loss • Down Syndrome • Epilepsy • Fetal Alcohol Spectrum Disorders (FASD) • Genetic Diseases: Overview • German Measles (Rubella) • Huntington Disease • Hydrocephalus • Lead Poisoning • Lou Gehrig's Disease (Amyotrophic Lateral Sclerosis) • Marfan Syndrome • Microcephaly • Phenylketonuria (PKU) • Prematurity • Radiation Exposure • Sickle Cell Anemia • Spina Bifida • Sudden Infant Death Syndrome (SIDS) • Syphilis • Tay-Sachs Disease • Toxoplasmosis
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March of Dimes. 1275 Mamaroneck Ave., White Plains, NY 10605. Telephone: 914-997-4488. Website: http://www.marchofdimes.org (accessed March 26, 2016).
National Human Genome Research Institute. Bldg. 31, Rm. 4B09, 31 Center Dr., MSC 2152, 9000 Rockville Pike, Bethesda, MD 20892-2152. Telephone: 301-402-0911. Website: http://www.genome.gov (accessed March 26, 2016).
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* fetus (FEE-tus) is the term for an unborn human after it is an embryo, from nine weeks after fertilization until childbirth.
* congenital (kon-JEH-nih-tul) means present at birth.
* genetics (juh-NEH-tiks) is the branch of science that deals with heredity and the ways in which genes control the development and maintenance of organisms.
* embryology (EM-bree-AH-logee) is the branch of biology that deals with the development of sperm and ova, fertilization, and the development of the fertilized ovum.
* nucleus is the part of the cell that contains its genetic information.
* chromosomes (KRO-mo-somz) are threadlike chemical structures inside cells on which the genes are located. There are 46 (23 pairs) of chromosomes in normal human cells.
* genes (JEENS) are chemical structures composed of deoxyribonucleic acid (DNA) that help determine a person's body structure and physical characteristics. Inherited from a person's parents, genes are contained in the chromosomes found in the body's cells.
* meiosis (my-OH-sis) is the process of reduction division in which the number of chromosomes per cell is cut in half.
* sickle cell anemia is a hereditary condition in which the red blood cells, which are usually round, take on an abnormal crescent shape and have a decreased ability to carry oxygen throughout the body.
* intellectual disability is a condition in which people have below-average intelligence that limits their ability to function in mainstream society.
* epilepsy (EP-i-lep-see) is a condition of the nervous system characterized by recurrent seizures that temporarily affect a person's awareness, movements, or sensations. Seizures occur when powerful, rapid bursts of electrical energy interrupt the normal electrical patterns of the brain.
* hydrocephalus (HY-droe-SEFuh- lus) is a condition, sometimes present at birth, in which there is an abnormal buildup of fluid within the skull, leading to enlargement of the skull and pressure on the brain.