A birth defect is a significant abnormality of appearance,
structure, or function that is present at birth. Birth defects are
common. Two percent to three percent of live-born infants show one
or more significant defects at birth. This number increases to
approximately five percent 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 hemagioma (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 percent 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.