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Birth Defects Exposure and Risk

Chromosomal Abnormalities

Chromosomal abnormalities include extra or missing genetic material visible under the microscope - cause unusual physical features, structural birth defects and mental retardation. Each cell contains pairs of chromosomes, one inherited from the mother and one from the father.

The most common chromosomal abnormality is Down syndrome. This syndrome results from an extra chromosome #21. It is also referred to as trisomy 21.

Congenital Heart Defects

In Iowa, 13 of every 1000 births result in an infant with a congenital heart defect. Many of these defects are relatively minor; however, malformations of the heart are the leading cause of premature death from birth defects. Three of the most common heart defects are Tetralogy of Fallot, Transposition of the Great Vessels and Ventricular Septal Defect.

Tetralogy of Fallot is the most common congenital heart defect that results in a "blue baby." This defect involves an obstruction to the usual blood flow through the heart and this results in blood that does not carry enough oxygen. Infants with Transposition of the Great Vessels have a reversal of the aorta and the pulmonary artery. This results in two separate circulation systems for the blood in the heart. The most common congenital heart defect is Ventricular Septal Defect. This defect is an abnormal opening between the two ventricles (pumping chambers) of the heart.

In most instances, the cause of the congenital heart defects is unknown, but in about 10% of children, a cause can be identified. These causes fall into one of four categories: chromosomal abnormalities, single gene defects, environmental factors and familial factors. Infants born to diabetic mothers, mothers who have rubella during pregnancy, mothers who drink alcohol while pregnant, mothers on anticonvulsant therapy or mothers exposed to some other drugs or chemicals at work or in the home, are all at increased risk of congenital heart defect in their infant. The greatest risk factor is a family history of a congenital heart defect in a parent or older sibling.

Neural Tube Defects

Neural tube defects are a group of disorders caused by failure of development of the structures which form and enclose the spinal cord and brain. These conditions are among the most common birth defects, affecting about 1 in 1,000 infants born in Iowa or about 40 cases per year. Approximately half of these defects are openings in the spine (spina bifida), which lead to a loss of sensation and muscle control below the opening. In the remainder of cases, the top of the brain and skull do not develop. This condition, called anencephaly, leads to death of the infant.

An insufficient level of folic acid is a risk factor for neural tube defects. Women who take a multivitamin containing folic acid every day can decrease their chance of having a child with a neural tube defect by up to 50%. The risk also decreases in women eating a diet with higher levels of folic acid. The neural tube closes within 28 days after conception, often before a woman is aware she is pregnant. For this reason it is important that folic acid be taken before beginning a pregnancy.

Cleft Lip and Cleft Palate

Clefts of the lip and/or palate are caused by disturbances in the embryo or fetus usually between 6 and 12 weeks after conception.

Clefts of the lip typically occur in the space stretching from one or both nostrils to the upper part of the lip. Clefts of the palate, or the roof of the mouth, can occur in either the front part of the palate, which has underlying bone, or in the back portion of the mouth, the so-called 'soft palate.'

Risk factors for clefts of the lip and/or palate include environmental factors with evidence already being documented that alcohol and smoking can contribute to such clefts. In addition, genetic factors can be identified in some specific families. A family history of a cleft lip and/or palate, there may be an increased risk of a cleft in the fetus.

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This effort is supported by funding from the U.S. Centers for Disease Control and Prevention Environmental Public Health Tracking Program, Cooperative Agreement Number 5U38EH000619-02. The contents of this Website are solely the responsibility of the authors and do not necessarily represent the official views of the Centers for Disease Control and Prevention.