What are birth defects?

A birth defect is a significant abnormality of appearance, structure, or function that is present at birth. Birth defects are common. Two percent to 3% of live-born infants show one or more significant defects at birth. This number increases to approximately 5% by 1 year of age due to the discovery of defects that were not obvious at birth.

A birth defect can be visibly obvious, an internal defect, or a chemical imbalance within the body:

  • An example of a visibly obvious defect could be the absence of an arm or a hemangioma (blood vessel birthmark) on the surface of the skin.
  • An internal defect could be a malformed kidney or a ventricular septal defect (a hole between the lower chambers of the heart).
  • An example of a chemical imbalance would be phenylketonuria (a defect in a chemical reaction of the body that results in severe mental retardation).

An individual can have a single birth defect such as cleft lip (a gap in the upper lip) or multiple birth defects such as cleft lip and cleft palate (a hole in the roof of the mouth) together, or even cleft lip and cleft palate with associated birth defects of the brain, heart, and kidneys.

Not all birth defects are detectable at birth. Some, such as sickle cell anemia (a defect in red blood cells that causes severe anemia and bone pain) might not become apparent until the child is several months old. A malformed kidney might take years to be discovered.

Some birth defects might remain silent for many years. For example, Huntington disease is an inherited condition in which affected persons live normal lives for decades. Then, during middle age, they develop dementia and loss of control of their arms and legs.

What are the causes of birth defects?

Birth defects can be caused by genetic factors and by a variety of environmental injuries such as infection, radiation, and drug exposure during pregnancy. The majority of birth defects, however, are without detectable cause.

About 20% of birth defects are caused by genetic or hereditary factors. Every human body cell contains 46 chromosomes, and each chromosome contains thousands of genes. Each gene contains a blueprint that controls development or function of a particular body part. People who have either too many or too few chromosomes will therefore receive a scrambled message regarding body development and function.

Down syndrome is an example of a condition caused by too many chromosomes. Because of an accident during cell division, individuals with Down syndrome have an extra copy of a particular chromosome. This extra chromosome causes a typical group of birth defects to occur repeatedly in affected persons. Individuals with Down syndrome have mental retardation, muscle weakness, downward slant of the eyes, low-set and malformed ears, an abnormal crease in the palm of the hand, and birth defects of the heart and intestines.

With Turner syndrome, a disorder that s only women, a person lacks a particular chromosome. In the affected persons, this causes short stature, learning disabilities, and absence of ovaries.

Since each of the genes on the chromosomes controls the structure and function of a body part, people with defective genes will show defects in the corresponding body part. These abnormal genes often cause hereditary disorders that pass down within a family. Hemophilia (absence of a blood-clotting chemical) is a familial disorder caused by a defective gene. Other examples of familial gene disorders are cystic fibrosis (a disorder that causes progressive damage of the lungs and pancreas) and sickle cell anemia.

Defective genes can also be caused by accidental damage, a condition called spontaneous mutation. Most cases of achondroplasia (a condition that causes extreme short stature and malformed bones) are caused by new damage to the controlling gene.

Other causes

About 10% of birth defects are caused by environmental factors such as infection, radiation, and drugs. These environmental factors can cause death, severe birth defects, or might have no effect at all on the developing baby depending on when during pregnancy the exposure occurs.

The developing baby goes through two major stages of development after conception. The first, or embryo stage, occurs during the first 10 weeks after conception. Most of the major body systems and organs form during this time. The second, or fetal stage, is the remainder of the pregnancy. This fetal period is a time of growth of the organs and of the fetus in general. The developing baby is most vulnerable to injury during the embryo stage when organs are developing. Indeed, infections, radiation, and drugs cause most of their damage when exposure occurs 2 to 10 weeks after conception.

Rubella, or German measles, is an example of an infection that causes birth defects if a woman has the infection during early pregnancy. The type of birth defect that results depends on the stage of development when the infection occurs. Rubella causes cataracts if infection occurs during the 6th week of pregnancy, deafness if the infection occurs during the 9th week, and heart defects if the infection occurs between the fifth and 10th week of pregnancy.

Radiation during the first 3 months of pregnancy can cause birth defects such as microcephaly (small head), spina bifida (a hole in the back that marks the premature end of the spinal nerves), blindness, or cleft palate. Various medicines and recreational drugs can cause birth defects, which are most severe when used during the first 3 months of pregnancy. Thalidomide, an anti-nausea medicine prescribed during the 1960s, caused birth defects called phocomelia (absence of most of the arm with the hands extending flipper-like from the shoulders).

Alcohol is the most commonly used drug that causes birth defects. Fetal alcohol syndrome is a term used to describe the typical birth defects (learning disabilities, mental retardation, irritability, hyperactivity, poor coordination, and abnormalities of facial features) caused by maternal alcohol use. This early vulnerability of the developing baby has serious consequences. A woman might not be aware that she is several weeks pregnant and might unknowingly expose her unborn child to the effects of medicines, X-rays, recreational drugs, or alcohol during this most vulnerable period.

Another kind of environmental factor that causes birth defects is uterine constraint. The human fetus grows in its mother’s uterus and is surrounded by amniotic fluid (similar to being suspended in a bag of water) that cushions it from excessive pressure. If the sack of fibers that holds the fluid breaks, bands of fibers from the torn sack can press on the fetus and cause amniotic band syndrome (amputation of an arm or leg). An inadequate amount of amniotic fluid can cause excessive pressure on the entire baby, causing pulmonary hypoplasia (lack of development of the lungs).

Medical science has identified the cause of about 30% of birth defects. That means about 70% remain without a straightforward cause. These difficult-to-classify birth defects have multi-factoral causes, or the causes are simply unknown. Multi-factoral means that the defect is caused by a complicated combination of both genetic and environmental factors.

For example, some cases of spina bifida are caused by inadequate folic acid intake by pregnant women who have a mild deficiency in a particular chemical reaction of the body. The birth defect does not occur in the offspring of women with the defective chemical reaction if they take extra folic acid before and during the pregnancy; the defect occurs in the offspring of women who both have the defective chemical pathway and who do not take folic acid supplements.

Last reviewed by a Cleveland Clinic medical professional on 08/17/2012.

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