Birth Defects
Birth defects, or congenital
malformations, are the faulty formation of
structures or body parts present at birth.
Sporadic, hereditary, or acquired defects may be
immediately observed or may become manifest
later in life; they may be visible on the body
surface or present internally. Birth defects may be
life threatening and require surgical correction, or
they may interfere with function or appearance. It
is estimated that about 3% of all children are born
with major defects; minor defects or variations are
estimated to occur in 10% to 15% of births.
Malformations may be single or multiple. Multiple
malformations that occur in a regular recognizable
pattern are referred to as syndromes--for
example, the FETAL ALCOHOL SYNDROME
sometimes observed in infants of mothers who
drank heavily when pregnant. Birth defects may
result from the action of genes, chromosomes, or
the environment on the developing fetus, but often
the cause cannot be determined. Inherited Defects
Abnormal genes cause a significant number of
different birth defects. Some can be identified as a
single-gene disorder that is inherited in a simple
Mendelian mode, that is, either a dominant or a
recessive pattern. For example, lobster claw
deformity of the hands and feet (split hands or
feet) is inherited and results from the effect of a
single dominant gene. A person who has this
deformity runs a 50% risk (1 in 2) of bearing
offspring who will inherit the gene and will
therefore also be affected. Autosomal recessive
inheritance and X-linked recessive inheritance
account for the other forms of single-gene
inheritance that cause birth defects. In cases of
autosomal recessive inheritance, both parents are
normal but each carries a silent, or recessive, gene
that, if matched in an offspring, causes the birth
defect. Because both parents are so-called
carriers (heterozygotes) of the same abnormal
gene, they run a 25% risk (1 in 4) of having a child
with the birth
malformations, are the faulty formation of
structures or body parts present at birth.
Sporadic, hereditary, or acquired defects may be
immediately observed or may become manifest
later in life; they may be visible on the body
surface or present internally. Birth defects may be
life threatening and require surgical correction, or
they may interfere with function or appearance. It
is estimated that about 3% of all children are born
with major defects; minor defects or variations are
estimated to occur in 10% to 15% of births.
Malformations may be single or multiple. Multiple
malformations that occur in a regular recognizable
pattern are referred to as syndromes--for
example, the FETAL ALCOHOL SYNDROME
sometimes observed in infants of mothers who
drank heavily when pregnant. Birth defects may
result from the action of genes, chromosomes, or
the environment on the developing fetus, but often
the cause cannot be determined. Inherited Defects
Abnormal genes cause a significant number of
different birth defects. Some can be identified as a
single-gene disorder that is inherited in a simple
Mendelian mode, that is, either a dominant or a
recessive pattern. For example, lobster claw
deformity of the hands and feet (split hands or
feet) is inherited and results from the effect of a
single dominant gene. A person who has this
deformity runs a 50% risk (1 in 2) of bearing
offspring who will inherit the gene and will
therefore also be affected. Autosomal recessive
inheritance and X-linked recessive inheritance
account for the other forms of single-gene
inheritance that cause birth defects. In cases of
autosomal recessive inheritance, both parents are
normal but each carries a silent, or recessive, gene
that, if matched in an offspring, causes the birth
defect. Because both parents are so-called
carriers (heterozygotes) of the same abnormal
gene, they run a 25% risk (1 in 4) of having a child
with the birth
Bibliography: Garell, Dale C., and Snyder, Solomon H., eds., Birth Defects (1989); Jones, Kenneth L., Smith 's Recognizable Patterns of Human Malformation, 4th ed. (1988); Pergaud, T. V., Environmental Causes of Human Birth Defects (1990).