|Classification and external resources|
|This article needs attention from an expert in Medicine. (October 2009)|
A congenital anomaly (congenital abnormality, congenital malformation, birth defect) is a condition which is present at the time of birth which varies from the standard presentation.
A limb anomaly is called a dysmelia. These include all forms of limbs anomalies, such as amelia, ectrodactyly, phocomelia, polymelia, polydactyly, syndactyly, polysyndactyly, oligodactyly, brachydactyly, achondroplasia, congenital aplasia or hypoplasia, amniotic band syndrome, and cleidocranial dysostosis.
Congenital anomalies of the heart include 
Congenital anomalies of the nervous system include neural tube defects such as spina bifida, meningocele, meningomyelocele, encephalocele and anencephaly. Other congenital anomalies of the nervous system include the Arnold-Chiari malformation, the Dandy-Walker malformation, hydrocephalus, microencephaly, megencephaly, lissencephaly, polymicrogyria, holoprosencephaly, and agenesis of the corpus callosum.
Congenital anomalies of the kidney and urinary tract (CAKUT) include renal parenchyma, kidneys, and urinary collecting system.
Defects can be bilateral or unilateral, and different defects often coexist in an individual child
About 3% of newborns have a "major physical anomaly", meaning a physical anomaly that has cosmetic or functional significance.
Congenital anomalies involving the brain are the largest group at 10 per 1000 live births, compared to heart at 8 per 1000, kidneys at 4 per 1000, and limbs at 1 per 1000. All other physical anomalies have a combined incidence of 6 per 1000 live births.
Congenital anomalies of the heart have the highest risk of death in infancy, accounting for 28% of infant deaths due to congenital anomaly, while chromosomal anomalies and respiratory anomalies each account for 15%, and brain anomalies about 12%.
The cause of 40-60% of congenital anomalies in humans is unknown. These are referred to as sporadic, a term that implies an unknown cause, random occurrence regardless of maternal living conditions, and a low recurrence risk for future children. For 20-25% of anomalies there seems to be a "multifactorial" cause, meaning a complex interaction of multiple minor genetic anomalies with environmental risk factors. Another 10-13% of anomalies have a purely environmental cause (e.g. infections, illness, or drug abuse in the mother). Only 12-25% of anomalies have a purely genetic cause. Of these, the majority are chromosomal anomalies.
Environmental causes of congenital anomalies are referred to as teratogenic. These are generally problems with the mother's environment. Teratogens can include dietary deficiencies, toxins, or infections. For example, dietary deficiency of maternal folic acid is associated with spina bifida. Ingestion of harmful substances by the mother (e.g., alcohol, mercury, or prescription drugs such as phenytoin) can cause recognizable combinations of birth defects.
The greatest risk of a malformation due to environmental exposure to a teratogen (terato = monster, gen = producing) between the third and eighth week of gestation. Before this time, any damage to the embryo is likely to result in fatality, and the baby will not be born. After eight weeks, the fetus and its organs are more developed and less sensitive to teratogenic incidents.
The type of congenital anomaly is also related to the time of exposure to a teratogen. For instance, the heart is susceptible from three to eight weeks, but the ear is susceptible from a slightly later time to about twelve weeks.
Many common skin care ingredients can be absorbed through the skin in small amounts, where they can enter the bloodstream and pass into the fetus's circulation.
Infection in the mother early in the third week may cause fetal cardiac damage. An infection in the eleventh week is less likely to damage the heart, but the baby may be born deaf. A common cause of congenital deafness of children in particular is a measles infection in the mother.