Abdominal wall defects: incomplete development of the skin and muscles of the belly, allowing internal organs to protrude into the umbilical cord (omphalocele) or elsewhere (gastroschisis).

Agenesis: failure to develop; absence.

Amniocentesis: a prenatal test where amniotic fluid is withdrawn using a needle inserted into the uterus. The fluid and the cells it contains can be used to perform tests on the fetus, including chromosome testing. Usually performed in the 2nd trimester.

Anal atresia: incomplete development of the lowest part of the large intestine, preventing passage of stool (feces).

Anencephaly: neural tube defect where the brain and skull do not form completely. Affected babies die, either before birth (stillbirth) or shortly thereafter. Because the brain is only partly formed, affected newborns cannot control basic life functions like breathing.

Aortic artery/valve defect: heart defect involving the aorta (the main blood vessel leading from the heart). Defects include narrowing or complete closure of the vessel (artery) or valve (connection between the heart and the aorta).

Atresia: absence or blockage of an opening such as the intestinal tract.

Atrioventricular canal: heart defect common in Down syndrome where the intersection between the heart's 4 chambers (the atria and the ventricles) doesn't close completely. Also called endocardial cushion defect.

Autism: a developmental disability that can cause problems with communication, social interaction and routine/repetitive behaviors. Although not diagnosed at birth, autism probably has origins in prenatal development and therefore is a type of birth defect.

Autosomal dominant: inheritance pattern where only 1 of the gene pair is needed to cause the trait in question. The disease-causing gene can be inherited from an affected parent or arise as a new mutation. (Autosome means the gene is carried on a non-sex-determining chromosome1 of 22 chromosome pairs that are the same in males and females.)

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Bias: potential source of error in epidemiology studies. If people with/without the risk factor are more likely to be included in a study, results may not be impartial.

Bilateral: occurring on both sides. For example, bilateral limb defects affect both arms or both legs.

Birth defects: conditions resulting from faulty prenatal development. Structural birth defectsthose affecting the bodyare usually apparent in the first year of life. Functional problems in brain development (leading to learning problems or mental retardation) may not be apparent until childhood.

Birth defects association: a group of birth defects that occur together more frequently than predicted by chance. For example, the VATER association includes Vertebral defects, Anal atresia, Tracheo-Esophageal defects and Renal (kidney) problems. An association can have a variety of underlying causes (chromosome defects, for instance) or the cause may be unknown.

Birth defects sequence: a group of birth defects caused by a single underlying event. For example, when the fetus does not urinate because of faulty kidney development, the lack of amniotic fluid (formed from fetal urine) restricts chest/lung growth and causes loose skin as well as club feet. These are all secondary characteristics of the sequence; the primary event is the abnormal kidney development.

Birth defects syndrome: a characteristic pattern of physical findings. Down syndromeassociated with a chromosome abnormalityis an example. There are many recognized syndromes where causes are unknown, however.

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Cardiac defects: see heart defects.

Case-control study: epidemiology study that uses a sample group of people rather than the entire population. It compares characteristics of those with birth defects (cases) and those without (controls). The key question is: Does a specific exposure occur more in cases than in controls?

Central nervous system: the brain and spinal cord.

Cerebral palsy: condition where the brain does not properly control muscles and movement. Some people with cerebral palsy have additional disabilities, such as mental retardation or seizures. Although not a structural birth defect, cerebral palsy may have its origins in prenatal development and is sometimes accompanied by structural birth defects.

Chorionic villus sampling (CVS): a prenatal test usually done in the 1st trimester to detect fetal abnormalities. A small piece of the developing placenta is withdrawn; because this has the same genetic make-up as the developing fetus, it can be used for genetic or chromosome tests.

Chromosome/chromosome abnormalities: packages containing the genes, located in every cell of the body. Normally, humans have 46 chromosomeshalf come from the mother and half from the father. Extra or missing chromosomesor even parts of chromosomesgenerally result in multiple birth defects and mental retardation. Trisomy47 chromosomes instead of the usual 46is the most common type.

Cleft lip: incomplete development of the lip, usually occurring on the upper lip. The split in the lip can occur on either or both sides (unilateral or bilateral cleft) and may be accompanied by a cleft palate. (This condition was formerly called "harelip.")

Cleft palate: incomplete development of the roof of the mouth. It can occur alone or accompanied by a cleft lip.

Cluster investigation: systematic evaluation of an unusual number or presentation of birth defects cases.

Colorectal atresia: abnormally formed segments in the colon or rectum (large intestine) block food movement during digestion; besides obstructions, there are often abnormal connections between the intestines and genitourinary tract. Surgery must be done at birth to remove the underdeveloped areas and reconnect the bowel. (See intestinal atresia, anal atresia.)

Confidence interval: calculation that helps show the statistical precision of a rate. It shows the most likely range of the "true" ratea wide confidence interval indicates the uncertainty stemming from small numbers; a narrow interval reflects a high degree of statistical confidence.

Confounding: epidemiologic term describing characteristics closely linked to the exposure under study, possibly leading to incomplete or incorrect conclusions. For example, maternal obesity is linked to neural tube defects. Does obesity cause neural tube defects directly? Or is a linked factorsuch as nutrition, altered metabolism or underlying genetic defectresponsible?

Congenital abnormality: a problem present at birth (see birth defects).

Conotruncal heart defects: heart defect occurring early in development, involving faulty septation/connection of the heart's chambers and/or the major blood vessels leaving the heart.

Craniofacial: involving the face and skull.

Craniosynostosis: too early closure of the skull bones, causing unusual head shape.

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Demographics: population characteristics such as race/ethnicity, age, education level, residence.

Developmental disabilities: problems caused by delayed mental development, such as mental retardation.

Diabetes: chronic illness causing high blood sugar levels; often treated with the drug insulin.

Diaphragmatic hernia: incompletely formed diaphragm (muscular sheet dividing the chest and abdomen) allows the stomach, liver and other abdominal organs to move into the chest. This crowds the developing lungs, which may be too small to support breathing after birth, even if the diaphragm is surgically repaired.

Dilantin: an anticonvulsant (seizure medication) that can cause birth defects.

DNA: molecular (biochemical) basis for the genedeoxyribonucleic acid.

Down syndrome: condition where an extra chromosome 21 causes distinctive physical features and mental retardation; also called trisomy 21. (This condition was formerly called "mongolism.")

Duodenal atresia: lacking a functional connection in the upper part of the intestine where it meets the stomach.

Dysgenesis: abnormally formed organ or body structure.

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Endocardiocushion defect: see atrioventricular canal.

Environment: surroundings. In research on prenatal development, this refers to the fetal surroundings and includes anything the expectant mother may come into contact with during pregnancy (the food she eats, the water she drinks, illnesses and medications, work exposures and so on).

Epidemiology: study of disease occurrence patterns in the human population to look for causes; measuring the relationship between an exposure and an outcome (in this case, birth defects).

Esophageal atresia: condition where the upper and lower ends of the esophagus (the swallowing tube leading from the mouth to the stomach) are not connected. Often associated with tracheal defects (involving the breathing tube connecting the mouth and lungs).

Expanded AFP blood test: a test done in the 2nd trimester of pregnancy. Levels of various chemicals (including Alpha FetoProtein) in the mother's blood tell whether the pregnancy is at higher or lower risk for fetal abnormalities (chromosome, neural tube and abdominal wall defects). Further testing (ultrasound, amniocentesis) can be done to determine if the fetus is affected.

Expected Rate: frequency of birth defects expected under typical conditions, based on studies of population averages. This serves as a reference point to compare with actual observations. For instance, birth defects occur in about 3% of newbornsthis is the "expected rate."

Exposure: contact with a particular substance.

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Fetal alcohol syndrome (FAS): physical findings and developmental disability caused by heavy drinking during pregnancy. Characteristics include: low birthweight, poor growth after birth, small head, small eyelid openings, smooth philtrum (the area between the nose and lip) and thin upper lip. Alcohol-exposed children with developmental delays but few of the physical features of FAS may be diagnosed with "fetal alcohol effects."

Folic acid: B vitamin found in green leafy vegetables and dried beans as well as in fortified cereal and flour. When taken around the time of conception and in early pregnancy, folic acid is associated with lower risk for neural tube defects, oral clefts, limb and heart defects.

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Gastroschisis: condition where the intestines protrude through a hole in the abdomen to the side of the umbilical cord; part of the category abdominal wall defects.

Gastrointestinal defects: defects along the gastrointestinal tract (the mouth to the anus), including defects of the stomach, intestines.

Gene: genetic blueprint for a single trait. A gene provides a template for the cell to make a specific protein. Controlling when and how much protein is made determines how the trait is expressed. Genes are inherited from the parents. A change in the gene's structure is called a mutation.

Genetic: involving DNA, the genes or the chromosomesthe physical structures that guide development and are the basis for inheritance. However, not all genetic errors are inheritedthey may be the result of a mutation or spontaneous change.

Genital defects: abnormalities of the reproductive/sex organs.

Genitourinary tract: external genitalia, internal reproductive organs and structures involved in collecting and carrying urine (bladder, urethra); these develop through related processes in early fetal life.

Genetic counselor: health professional specializing in birth defects and genetic disease, able to assess and discuss individual/family risk, recurrence risk and testing options.

Gestation: pregnancy timespan. For example, "20 weeks gestation" refers to the 20th week of pregnancy. The normal human gestation is 37-42 weeks.

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Heart defects: abnormal structures of the heart and/or large vessels leading from the heart; impairing distribution of oxygen and nutrients throughout the body. Also called cardiac defects.

Hirschsprung disease: a condition where sections of the intestine lack nerve stimulation; food is not moved through the gut, which becomes distended. Requires surgery to remove the faulty sections.

Holoprosencephaly: serious brain abnormality where the developing forebrain fails to undergo normal division into 2 lobes. Facial development is often alteredthere may be a single eye, closely spaced eyes with a single nostril, a midline cleft lip or a single central front incisor tooth. Holoprosencephaly often causes mental retardation and/or death.

Hydrocephalus: fluid accumulation in the brain, often due to a blockage in the canals that distribute fluid within the brain and spinal cord.

Hypospadias: birth defect of the penis where the urinary opening is misplaced (on the shaft or in the scrotum in the more serious forms).

Hypothesis: formal idea or theory tested in a scientific study.

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Imperforate anus: absence of an external opening from the rectum/intestinal tract. Surgery is needed to allow the infant to pass feces.

Infant mortality: deaths occurring in the first year after birth.

Intestinal atresia: condition where abnormally formed segments in the intestine obstruct food movement during digestion. Often classified according to the part of the gut affected: from the stomach to the colon (small intestine), the colon and rectum (large intestine) or anus. See colorectal atresia and anal atresia.

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Kidney defects: absence, underdevelopment or other structural abnormality of the kidneys. Also called renal defects.

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Limb defects: missing or malformed arms, legs, hands, fingers, feet or toes. Different types of limb defects result from different prenatal processes, so proper classification is essential.

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Malformation: see structural birth defects

Malrotation of the intestines: unusual configuration of intestinal (bowel) loops.

Maternal serum AFP screening: see expanded AFP blood test.

Mental retardation: abnormal brain function resulting in an IQ score of less than 70 (100 is average).

Metabolic disease: genetic disease caused by absent or defective protein. This disrupts metabolism at a cellular leveloften the cell cannot create, use or dispose of nutrients properlyleading to a variety of physical problems. PKU is an example where the inability to transform dietary protein leads to buildup of toxic waste and causes brain damage.

Miscarriage: pregnancy loss occurring before 20 weeks gestation (the first half of pregnancy). Also called spontaneous abortion.

Mutation: change in the DNA sequence of a gene, resulting in an altered expression of the trait that gene controls.

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Neural tube defects: abnormalities arising during early fetal development when the neural tubethe precursor of the spinal cord and braindoes not form correctly. Can cause absence of the brain (anencephaly) or spina bifida (open spine defect).

Nitrates: commonly occurring group of substances (found in foods, medications, drinking water and cigarette smoke) linked to cancer and other conditions.

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Omphalocele: condition where the abdominal wall is incompletely formed, allowing internal organs to protrude into the umbilical cord. Surgery to replace the organs can crowd the lungs, causing breathing problems. Omphalocele (pronounced om-FAL-oh-seel) is part of the category abdominal wall defects.

Open spine defect: see spina bifida.

Oral cleft: opening in the lip and/or palate occurring in early fetal development. Affected children need surgery to repair the cleft lip or cleft palate. They often need long-term help such as speech therapy.

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Polydactyly: extra finger or toe.

Pregnancy termination: Abortion; often done when birth defects are detected with prenatal diagnosis. Also called elective abortion.

Prenatal diagnosis: tests done during pregnancy to detect fetal abnormalities. Some tests are for screening, determining whether a pregnancy is at higher or lower risk, such as the expanded AFP blood test. Other testslike amniocentesisare diagnostic, telling for certain whether the fetus is affected.

Protective factor: characteristicsuch as taking multivitaminsassociated with a lower risk for birth defects. Usually it is not known if the characteristic is directly linked to the birth defect (a cause-and-effect relationship) or related to some other factor that's the actual cause.

Pulmonary artery/valve defect: type of heart defect involving the outflow tract leading from the heart to the lungs, with narrowing/closure of the blood vessel (pulmonary artery) or its connection to the heart (pulmonic valve).

Pyloric stenosis: overgrowth of the muscular connection between the stomach and intestines, blocking food passage and causing projectile vomiting a few weeks after birth.

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Radial defect: Underdeveloped or malformed radius, the larger of the 2 bones in the lower arm. This is often accompanied by defects in the surrounding tissue, especially the thumb, index and middle fingers.

Relative risk: comparison of the risk between exposed and unexposed persons. A relative risk of 1 means there is no difference. A relative risk of 10 means the outcome is 10 times more common among exposed people. A relative risk of 1/10 means the outcome is 10 times more frequent in unexposed people (the exposure is a protective factor).

Renal agenesis/dysgenesis: condition where kidneys are absent (agenesis) or severely underdeveloped or malformed (dysgenesis). If both kidneys are affected, a fetus does not urinate prenatally and there is little or no amniotic fluid (which comes from fetal urine). This in turn prevents the fetal lungs from growing and expanding, leading to breathing problems and possibly death soon after birth.

Renal defects: see kidney defects.

Risk factor: characteristicsuch as heavy drinkingassociated with a higher risk for birth defects. Usually it is not known if the characteristic is directly linked to the birth defect (a cause-and-effect relationship) or related to some other factor that's the actual cause.

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Single ventricle: major heart defect where the lower pumping chamber does not develop into 2 chambers (ventricles). Blood coming from the lungs carrying oxygen for the body is mixed with depleted blood returning from the body.

Small area study: systematic evaluation of birth defects occurrence in a small population such as a town, community or census tract.

Spina bifida: structural birth defect where the spine and spinal cord don't develop correctly. Depending on the size and location of the defect, it may cause paralysis or inability to control muscles in the legs or armsaffected children often use wheelchairs, walkers or braces. Hydrocephalus (back-up of spinal fluid in the brain) is a common side effect of spina bifida and sometimes results in developmental disabilities.

Stenosis: narrowing.

Structural birth defects: problems in prenatal development affecting the body structure, either externally (for example, cleft lip or missing limbs) or internally (such as heart defects, kidney defects). Also called malformations.

Syndrome: see birth defects syndrome.

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Tetralogy of Fallot: major defect of the heart's chambers and blood vessels leading to and from the heart. The 4 components are: underdeveloped right heart, hole between the lower heart chambers, overriding aorta and underdeveloped pulmonary artery.

Teratogen: an exposure that interferes with normal fetal development, causing a birth defect. Known teratogens include certain illnesses in the mother (German measles, diabetes), medications (thalidomide, Dilantin) and other exposures (excessive alcohol).

Thymus: gland in the neck that produces immune system cells.

Tracheo-esophageal fistula: Connection between the breathing tube (trachea) and swallowing tube (esophagus) causing life-threatening symptoms soon after birth. Surgery is needed to properly attach the trachea to the lungs and the esophagus to the stomach.

Transposition of the great vessels: major heart defect where the main blood vessels leading from the heart are reversed, resulting in improper circulation of oxygenated and depleted blood.

Triphalangeal thumb: a thumb with 3 bones/2 joints, resembling a finger.

Trisomy: 3 copies of a chromosome instead of the usual pair, generally resulting in structural birth defects and mental retardation. Humans normally have 23 pairs of chromosomes, numbered 1-22 plus the sex-determining pair. The most common trisomy is an extra 21 chromosome (trisomy 21 or Down syndrome); trisomy 13 and 18 are also seen.

Truncus arteriosus: a conotruncal heart defect where a single blood vessel replaces the pulmonary artery (leading to the lungs) and the aorta (leading to the rest of the body); this results in mixing of oxygenated and depleted blood.

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Ultrasound: prenatal test using sound waves to create a picture of the developing fetus. Ultrasound can measure growth and examine body structures such as the heart and spine.

Unilateral: occurring in 1 of paired organs/body structures. For example, a unilateral kidney defect affects a single kidney.

Urethra: the tube leading from the bladder to outside the body.

Urinary defects: defects of the structures for collecting and excreting urine. This includes the bladder, the tubes connecting the kidneys to the bladder (ureters) and the tube leading from the bladder to outside the body (the urethra).

Urinary tract obstruction: defect in the tubes leading to and from the bladder, preventing normal urination. Backed-up urine damages the kidneyschildren often need corrective surgery and possibly dialysis (mechanized help with filtering waste from the blood). In severe cases, the absence of amniotic fluid (which comes from fetal urine) prevents the fetal lungs from growing and expanding; once born, these babies cannot breathe and soon die.

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VATER association: combinations of defects occurring more frequently than expected. The VATER association is named for Vertebral/rib defects, Anal atresia, Tracheo-Esophageal fistula, Radial and Renal defects; adding Cardiac and Limb defects forms the VACTERL association.

Ventricular septal defect: heart defect where a hole between the two bottom pumping heart chambers (ventricles) allows oxygenated and unoxygenated blood to mix.

Vertebral defects: abnormalities of the spinal bones.

Vital Statistics: official records kept on individuals, particularly birth and death certificates.

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