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 A birth defect is an abnormality, present at birth, of the structure, function, or metabolism of a part of the body. According to various researches. Almost 150,000 babies who are born each year are sayed to born with a birth defect. In addition are more than 4,000 known birth defects, which, when taken together, are the leading cause of infant death.

Causes of Birth Defects.

Although the causes of the  most birth defects are unknown, many are attributable to a combination of some factors. Some birth defects are the result of genetic determinants, like  an abnormality due to an inherited trait. For instance, researchers have linked various physical malformations, metabolic abnormalities, certain vision and hearing losses, and other birth defects to specific genes that are inherited from one (or in rare cases, both) parent.

Problems may also arise from defects in a gene or chromosome structure or number. Down syndrome, which may lead to mental retardation, cardiac difficulties, and other problems, is caused by an extra copy of chromosome 21. As one of the most common serious birth defects, Down syndrome affects 1 in 900 births, and there is a substantially increased risk of giving birth to a child with Down syndrome if the mother is over thirty-five years of age.

Myriad environmental, or non-genetic factors have also been linked to birth defects. Prescription and nonprescription medications, illicit drugs, and other harmful chemicals can cause newborn abnormalities. In addition alcohol use during pregnancy has been linked to fetal alcohol syndrome, which occurs about once in every 1,000 births. Infants with fetal alcohol syndrome are born with a range of preventable physical and mental abnormalities.

Several birth defects can be traced to a mutation in a single gene or chromosome (e.g., neurofibromatosis type 1 and cystic fibrosis) or environmental influence (e.g., thalidomide, rubella virus, and ionizing irradiation), but most are due to a combination of these factors. This is referred to as multi factorial inheritance. Neural tube defects and orofacial clefts are two types of anomalies that are thought to have a multi factorial cause in most instances. Cleft lip, which results from an incomplete development of the lip, and cleft palate, which is an incomplete development of the roof of the mouth, may occur singly or in combination with each other.

 Cleft lip with or without cleft palate occurs more often than cleft palate alone, but infants with cleft palate alone are much more likely to have birth defects that involve other organ systems and are more likely to have chromosomal anomalies. Although these conditions can be remedied through surgery, speech and hearing difficulty may be associated with cleft palate.

The complexity of the causes of these birth defects are apparent in that they are associated with environmental factors such as maternal alcohol consumption, which has been observed at higher rates among Native Americans and Caucasians and relatively low rates in African Americans, and that there is increased risk for infants born to a parent with a cleft lip and/or palate.

Heart defects, the most common type of birth defect, affect about 25,000 infants each year and are considered to have a multi factorial genesis. Because of improvements in diagnostic techniques such as echocardiography, the number of infants diagnosed with heart defects has increased dramatically in the 1980s and 1990s. Heart defects vary greatly in severity and can occur in isolation or can be one component of a complex syndrome

.Malformations of the heart, such as atrial septal defects or ventricular enlargement, may be a result of using alcohol or certain medications during pregnancy. Mutations in certain genes have also been reported to cause some of the defects. Some malformations can be repaired with surgery. Although these types of birth defects are not completely preventable, a pregnant woman can reduce risk by discussing medications she is using with her doctor and by avoiding alcohol.

Prevention of Birth Defects

 In the past ten years, there have been significant strides in understanding ways to prevent some birth defects. For example, a daily supplement to the diet of 500 micrograms of folic acid, a B vitamin, has been shown to prevent up to 70% of cases of neural tube defects.

 Neural tube defects, which include anencephaly, spina bifida, and encephalocele, are serious and often lethal birth defects of the spine and central nervous system. The recognition that many of these birth defects can be prevented with folic acid has led to initiatives at the state and national levels aimed at educating women about the importance of consuming the appropriate amount of this vitamin on a daily basis. In 1996 the U.S. Food and Drug Administration issued a rule (effective January 1, 1998) requiring that all enriched grain products sold in the United States be fortified with 140 micrograms of folic acid per 100 grams of product.

Because several birth defects are caused by infections, prevention initiatives also emphasize immunization and information. For example, because of widespread vaccination for , the birth defects caused by this infection rarely occur in the United States. Information about the risk of birth defects resulting from maternal infection with syphilis or other sexually transmitted diseases may stimulate the development of services to help women at greatest risk.

Cytomegalovirus, the most common of the congenital viral infections, affects almost 40,000 infants each year. It can be passed through bodily fluids, such as saliva, blood, and breast milk. It is often passed to a pregnant woman from a child who is infected but is not showing symptoms; for example, an infected child may sneeze and then touch a pregnant woman, thus infecting her.

An infant born to a mother who has contracted cytomegalovirus is at an increased risk for mental retardation and vision or hearing loss. Although many types of birth defects are preventable, prevention is complicated by the fact that most serious birth defects occur during the early weeks of pregnancy, often before a woman even knows she is pregnant.

This is why strategies aimed at preventing birth defects must focus on improving the health of women prior to pregnancy. Screening and diagnostic tests, such as ultrasound, maternal serum a-fetoprotein screening, amniocentesis, and chorionic villus sampling, are used to monitor the health of the fetus and to identify certain fetal malformations and chromosomal disorders; they cannot, however, be used to prevent these conditions from occurring.

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