What are congenital defects? |

Normal Development

In order to understand the causes of birth defects, it is necessary to have some understanding of the stages of normal development. If the time and sequence of development of each organ are not correct, an abnormality may result. It has been useful to divide human pregnancy
into three major periods: the preembryonic stage, the embryonic stage, and the fetal stage.

The preembryonic stage is the first two weeks after fertilization. During this stage, the fertilized egg undergoes cell division, passes down the Fallopian tube, and implants in the uterine wall, making a physical connection with the mother. It is of interest to note that perhaps as many as one-half of fertilized eggs fail to implant, while half of those which do implant do not survive the second week. The second stage, the embryonic stage, runs from the beginning of the third week through the end of the eighth week. There is tremendous growth and specialization of cells during this period, as all of the body’s organs are formed. The embryonic stage is the time during which most birth defects are initiated.

The fetal stage runs from the beginning of the ninth week to birth. Most organs continue their rapid growth and development during this final period of gestation leading up to birth. By the end of the eighth week, the embryo, although it has features of a human being, is only about 1 inch (2.54 centimeters) long. Its growth is amazing during this period, reaching 12 inches (30 centimeters) by the end of the fifth month and somewhere around 20 inches (50 centimenters) by birth. It is evident from the description of normal development that the changes the embryo and fetus undergo are very rapid and complicated. It is not unexpected that mistakes can happen, leading to congenital disorders.

Causes of Birth Defects

Throughout history, examples of birth defects have been described by all cultures and ethnic groups. Although the incidence of specific malformations may vary from group to group, the overall incidence of birth defects is probably similar in all people on earth. According to 2014 data published by the World Health Organization, an estimated one in thirty-three infants has some sort of major or minor congenital disorder.

Humans have long sought an explanation for why some couples have babies afflicted with serious birth defects. Such children were long regarded as “omens” or warnings of a bad event to come. The word teratology (Greek for “monster causing”) was coined by scientists to reflect the connection of “monster” births with warnings. Frequently, ancient people sacrificed such babies. It was thought that such pregnancies resulted from women mating with animals or evil spirits. Maternal impression has long been invoked as an explanation for birth defects, and from early Greek times until more recent times, stories and superstitions abounded.

Of the birth defects in which a specific cause has been identified, it has been found that some are caused by genetic abnormalities, including gene mutations and chromosomal changes, while others are caused by exposure of the pregnant woman and her embryo or fetus to some sort of environmental toxin, such as radiation, viruses, drugs, or chemicals.

Examples of Birth Defects

Many birth defects are caused by changes in the number or structure of chromosomes. The best-known chromosomal disorder is Down syndrome, which results from individuals having an extra chromosome 21, giving them forty-seven chromosomes rather than the normal forty-six. A person with Down syndrome characteristically has a flattened face, square-shaped ears, epicanthal folds of the eye, a short neck, poor muscle tone, slow development, and below-average intelligence. Cystic fibrosis
is an example of a defect caused by a single gene. Affected people inherit a recessive gene from each parent. The disorder is physiological in nature and results in a lack of digestive juices and the production of thick and sticky mucus that tends to clog the lungs, pancreas, and liver, at times encouraging the development of potentially fatal respiratory infections. Cleft lip, or cleft palate, is multifactorial in inheritance (some cases are caused by chromosomal abnormalities, while others are the result of single-gene mutations). Multifactorial traits are caused by many pairs of genes, each having a small effect, and are usually influenced by factors in the environment. The result is that such traits do not follow precise, predictable patterns in a family.

Genetic factors account for the great majority of the birth defects in which there is a known cause. The remaining cases of known cause are attributed to maternal illness; congenital infections; exposure to chemicals, drugs, and medicines; and physical factors such as x-rays, carbon dioxide, and low temperature. The government warning on liquor bottles informs pregnant women that if they drink alcohol during a sensitive period of prenatal development, they run the risk of having children with fetal alcohol syndrome. There is a wide variation in the effects of alcohol on a developing fetus. Alcohol exposure can lead to an increased frequency of spontaneous abortion (or miscarriage), and it depresses growth rates both before and after birth. Facial features of a child exposed to alcohol may include eye folds, a short nose, small mid-face, a thin upper lip, a flat face, and a small head. These characteristics are likely to be associated with intellectual disabilities. Frequently, however, otherwise normal children have learning disorders and only a mild growth deficiency. Variation in the symptoms of prenatal alcohol exposure has made it difficult to estimate the true incidence of fetal alcohol syndrome. According to a 2014 report by the US Centers for Disease Control and Prevention, as many as 1.5 babies are born with fetal alcohol syndrome for every 1,000 live births in the United States.

For many babies born with a major birth disorder, no specific cause can be identified. Because of this rather large gap in knowledge, nonscientific explanations about the causes of birth defects flourish. What is known is that most congenital defects, whether caused by a genetic factor or an environmental factor, are initiated during the embryonic period. It is also known that some disorders, such as learning disorders, frequently result from damage to the fetus during the last three months of pregnancy. Knowledge about what can be done by parents to avoid toxic exposure and activity that could cause birth defects is critical.

Key Terms



sensitive period

:

a critical time during development when organs are most susceptible to teratogens





teratogen


:

any agent that is capable of causing an increase in the incidence of birth defects




teratology

:

the science or study of birth defects





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