Birth defects are any type of structural abnormalities that are present at birth.

The defect may be macroscopic or microscopic and on the surface or within the body.

The four clinically significant types of birth defect are                       ,                      ,                      ,                      .

Approximately    % of neonates have an obvious major defect.

Additional defects are detected after birth; the incidence is approximately 6% among 2-year-old children and 8% among 5-year-old children.

Other defects (approximately 2%) are detected later (e.g during surgery, dissection, autopsy)

Birth defects may be single or multiple and have minor or major clinical significance.

                        defects occur in approximately 14% of neonates. Theses defects have no serious medical consequences, but they alert clinicians to the possibility of an associated major defect.

Ninety percent of infants with multiple minor defects have one or more associated major defects.

Of the 3% of neonates with a major birth defect, multiple major anomalies are found in 0.7%.

Major defects are more common in                       (up to 15%) than they are in neonates(up to 3%)

Some birth defects are caused by                          (e.g. chromosomal abnormalities, mutant genes),

and a few defects are caused by                                       (e.g. infectious agents, environmental chemicals, drugs),

but most common defects result from a complex interaction between                                                 .

The cause of most birth defects is unknown.

During the                       of development, teratogenic agents usually kill the embryo or have no effects.

During the                                  , teratogenic agents disrupt development and may cause major birth defects.

During the                        , teratogens may produce morphologic and functional abnormalities, particularly of the brain and eye.