Information on Causes of Birth Defects

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Causes of Birth Defects

The underlying causes of many types of birth defects are understood, intwo out of three cases the specific reason is unknown and thus assumed to be the result of a complex interaction of genetic and nongenetic factors durir.,the intricate process of the baby's development within the womb. Birth defectswhich a specific cause can be identified fall into the following categories:

Genetic abnormalities

Genetic abnormalities, which arise directly from the transmission of genetic 7. -fo rm a ti on from one generation to the next, account for 20 to 25 percent ofbirth defects. In what is truly a marvel of engineering, all of the information ne.:-essary to form and maintain a human being is coded on long strands of the sr -ral molecule known as deoxyribonucleic acid, or DNA. An extended segmen.:DNA that serves a specific function (such as providing the blueprint for a certa -protein) is called a gene, and an estimated fifty to one hundred thousand of thegenes govern the physical attributes of each individual. Rather than being rar -domly dispersed throughout the cells whose functions it governs, DNA is packs :into forty-six dense bodies known as chromosomes, all of which easily fit the nucleus of a cell. Our forty-six chromosomes consist of twenty-three pairs. .which twenty-two arc known as nonsex or autosomal, while one pair consiststhe sex chromosomes designated X and Y.

Each of our 60 trillion cells contains all of our genetic code, with the excs: - tion of egg and sperm cells, which contain only a single set of twenty-threemosomes. At the time of conception, twenty-three chromosomes from the mother's egg and twenty-three from the father's sperm combine to begin ahuman being. These include an X chromosome contributed by the egg, andther an X or a Y chromosome from the sperm. If the final product contains -X chromosomes, the baby will be a girl, while an XY combination of chromsomes will result in a boy.

While this extraordinarily complex activity normally proceeds with:mishap, at times it does not function as originally designed. This might resul: -profound abnormalities in the number or structure of the chromosomes,that the fertilized egg may fail to divide properly, may not implant within the mother's uterus, or may not develop beyond a very early stage. In such cast:woman will have an early miscarriage or the new life will end without herbeing aware that it had started. However, a number of genetic abnormalities not end this way, and the pregnancy may continue to term. There are twotypes of genetic abnormalities:

Single-gene abnormalities involve a defect in a particular strand of DNA, whichturn causes a specific malfunction within the body. These abnormalities are -then subdivided into three categories: dominant, recessive, and X-linked These ariations arise from the fact that genes are paired—that is, the genes on a chro-no.osome inherited from the mother have a corresponding gene on the chromo-'me provided by the father. The effect of:le gene may override the other, or they might make an equal contribution to a particular trait.

A dominant genetic disorder (for example, Huntington's-chorea) needs only one (which can come from either parent) to be manifested. If one parent is affected, is a 50 percent chance that his or her (of either gender) will have the same::normality. Some dominant disorders runn families, while others result from a muta-- on that occurred in an individual egg or:erm.

A recessive genetic disorder (for example cystic fibrosis) is one that will occurilly when an abnormal gene that causes it is distributed by both parents. A child who has an abnormal recessive gene fromone parent usually will not manifest that defect, although he or she will be a carrier of the disorder. If both parents of a child are arriers of a recessive disorder, the child has a 50 percent chance of being a carrier, a -oercent chance of having two normal and a 25 percent chance of being affected. Recessive genetic defects often cluster in particular racial, ethnic, or geographic groups.

X-linked genetic disorders (for example,7-.Dphilia) are carried only on the X chromo-and may be dominant or recessive, although the latter are more common. Females an abnormal X-linked gene usually are not affected by the problem because they typically have a normal gene on their other X chromosome, which will override the abnormal gene.

Chromosomal abnormalities involve significantly larger amounts of genetic mate-rial than do single-gene defects. Either part or all of a chromosome maymissing or duplicated. Often the results are not compatible with developmer of the fetus within the womb, and a miscarriage results. (It is estimated tha:half of all miscarriages that occur during the first three months of pregnant:are caused by chromosomal abnormalities.) The abnormality may be passeaon from one or the other parent or may occur during the development of egzor sperm.

The risk of a child having a chromosomal abnormality increases stead:::with the age of his mother. The risk of a twenty-five-year-old woman havingbaby with Down syndrome—the most common disorder of this type—is 1 i::1,250, and the risk for all significant chromosomal disorders is 1 in 476. At acethirty-five, the numbers are 1 in 378 for Down syndrome and 1 in 192 for a::chromosomal disorders. At age forty-five, those numbers drop to 1 in 30 and 1in 21 respectively.'

Infections

Infections that are passed from mother to baby prior to birth account for about 3 to 5 percent of all birth defects. Medical problems in the mother, such as diabetes, high blood pressure, a heart disease, can affect the development of the baby growing within he-Detecting and managing any maternal conditions are key factors of prenatal Ca::and a major reason regular visits with a qualified physician are so important ing pregnancy.

Substances that pass from mother to baby

While the uterus, placenta, and amniotic fluid arc usually very effective in pro-viding a safe environment for the growing baby before birth, certain chemicalcompounds that enter the mother's bloodstream are capable of causing harm toher preborn child. The degree of damage can vary enormously depending uponthe amount of the substance involved and the stage(s) of development at whichthe exposure to the unborn child takes place. These substances include:

• Drugs—prescription and nonprescription, legal and illegal

• Alcohol and tobacco

• Chemical and occupational pollutants

The gamut of potential environmental exposures, especially in the workplace, is well beyond the scope of this book. However, if you are pregnantor might become so in the near future), you should become knowledgeableabout any chemical compounds to which you are exposed on a regular basis.

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