Birth Defects
Birth defects are defined as abnormalities of structure, function, or body metabolism that are present at birth. These abnormalities lead to mental or physical disabilities or are fatal. There are more than 4,000 different known birth defects ranging from minor to serious, and although many of them can be treated or cured, they are the leading cause of death in the first year of life.
According to the March of Dimes, about 150,000 babies are born with birth defects each year in the United States. The American College of Obstetricians and Gynecologists (ACOG) says that out of every 100 babies born in the United States, three have some kind of major birth defect.
Birth defects can be caused by genetic, environmental, or unknown factors.
Structural or metabolic defects are those that result when a specific body part is missing or formed incorrectly or when there is an inborn problem in body chemistry. The most common type of major structural defects are heart defects, which affect one in 100 babies in the United States. Some other common structural defects include spina bifida and hypospadias, a condition in which the opening of the male urethra (where urine exits from the penis) is in the wrong place. Metabolic defects affect one in 3,500 babies and usually involve a missing or incorrectly formed enzyme (one of the proteins necessary for processing chemical substances in the body). This type of defect can be harmful or even fatal, but doesn't usually cause any visible abnormalities in the child. Metabolic defects include Tay-Sachs disease, a fatal disease that affects the central nervous system, and phenylketonuria (PKU), which affects the way the body processes protein. Both of these defects will be discussed in more detail later in this article
Defects caused by congenital infections result when a mother gets an infection before or during the pregnancy. Examples of infections that can cause birth defects are rubella (German measles), cytomegalovirus (CMV), syphilis, toxoplasmosis, Venezuelan equine encephalitis, parvovirus, and, rarely, chicken pox. None of these infections affect 100% of babies whose mothers are infected during pregnancy. If the mother is infected during early pregnancy, rubella carries the highest risk for birth defects - approximately 20%.
What Causes Birth Defects?
Most babies with birth defects are born to two parents with no obvious health problems or risk factors. A woman can do everything her doctor recommends to deliver a healthy child and still have a baby with a birth defect.
In fact, according to the March of Dimes, about 60% of birth defects have unknown causes. The rest are caused by environmental or genetic factors, or some combination of the two.
Genetics play a role in some birth defects. Every cell in the body has chromosomes containing genes that determine a person's unique characteristics. One missing or faulty gene can cause a birth defect; this is significant when you consider that we each have about 25,000 to 35,000 genes per cell determining everything from the length of our toes to the color of our eyes.
Where do the faulty genes come from? A child inherits one of each pair of chromosomes (and one of each pair of the genes they contain) from each parent. Sometimes, a disease or defect can occur if only one parent passes along the gene for that disease (even though the child receives a normal gene from the other parent); this is called dominant inheritance and includes birth defects such as achondroplasia (a form of dwarfism) and Marfan syndrome (a disorder characterized by abnormally long fingers, arms, and legs). Some birth defects occur only when both parents (who are healthy) each pass along a faulty gene for the same disease to the child; this is called recessive inheritance and includes conditions such as Tay-Sachs disease or cystic fibrosis. Finally, some boys inherit disorders from genes passed on to them by their mothers. These defects, which include conditions such as hemophilia and color-blindness, are called X-linked because the genes are carried on the X chromosome. Because males have only the one X chromosome they receive from their mothers (females have two X chromosomes - one from each parent), a faulty gene on the X chromosome they receive will cause a problem because they don't have a normal copy of the gene on the other X chromosome that females have.
The number or structure of chromosomes can also cause birth defects. An error during the formation of an egg or sperm can cause a baby to be born with too few or too many chromosomes, or with a damaged chromosome. Birth defects caused by chromosome problems include Down syndrome. The risk of this type of birth defect often increases with the age of the mother.
Environmental causes of birth defects have more to do with the mother's health and exposure to chemicals or diseases. When a mother has certain infections, such as rubella, during pregnancy, it can cause birth defects. Alcohol abuse by the mother causes fetal alcohol syndrome, and certain medications taken by the mother can cause birth defects.
Multifactorial birth defects are caused by a combination of genetic and environmental factors and include neural tube defects and cleft lip and palate.
Although there are steps you can take to prevent birth defects, remember that a birth defect can happen even if you or your partner have no history of birth defects in your families or if you've had healthy children in the past.
Common Birth Defects
Cleft lip and/or palate occurs when the tissues of the mouth or lip don't form properly during fetal development. A cleft lip is a long opening between the upper lip and the nose. A cleft palate is an opening between the roof of the mouth and the nasal cavity. In the United States, oral clefting occurs in one in 700 to 1,000 births, making it one of the most common major birth defects. Clefts occur more frequently in children of Asian, Latino, or Native American descent; it's believed that a variety of environmental and genetic factors cause clefting, but it's unknown exactly which ones. Clefting is a birth defect that can be surgically repaired after birth.
Neural tube defects (NTDs) occur in the first month of pregnancy when the structure that develops into the brain and spinal cord is forming. Normally, this structure folds into a tube by the 29th day after conception. When the tube doesn't close completely, the baby has an NTD; many babies with these defects are stillborn or die soon after birth. The two most common forms of NTDs are:
- Spina bifida, which happens when the spinal column doesn't close completely around the spinal cord. It ranges from mild to severe and can be associated with loss of bladder and bowel function, paralysis, and, in some cases, death.
- Anencephaly, which occurs in three out of 10,000 births, involves the lack of development of parts of the brain.
- Atrial and ventricular septal defects, which are holes in the walls that separate the heart into left and right sides.
- Patent ductus arteriosus, which is present when the tubular blood channel that allows the blood to bypass the lungs while the baby is in the womb doesn't close (as expected) after birth.
- Aortic or pulmonary valve stenosis, which are narrowings of the valves that allow blood to flow from the heart to the lungs and other parts of the body.
- Coarctation of the aorta, which is a narrowing of the aorta, the major blood vessel carrying blood from the heart to the rest of the body.
- Transposition of the great arteries, which is the reversal of the connections of the aorta and the pulmonary artery with the heart.
- Hypoplastic left heart system, which is when the side of the heart that pumps blood to the body is underdeveloped or nonexistent.
- Tetralogy of Fallot, which is a combination of four heart defects that involves restriction in the flow of blood to the lungs.
Phenylketonuria (PKU) is a disease that affects the way the body processes protein; it can cause mental retardation. A baby born with PKU appears normal, but if the disease isn't treated, the child will suffer from developmental delays that are apparent by the time she's a year old. About one baby in 10,000 to 25,000 in the United States is born with PKU. This is a genetic disorder; if both parents carry the gene, there's a one in four chance a child will be born with it. Fortunately, it is usually detected within a few days after birth by a screening blood test that is routinely performed on infants in the United States and other developed countries. If the child is promptly treated with a special diet, retardation can be prevented.
Fragile X syndrome is characterized by mental impairment, ranging from learning disabilities to mental retardation, autistic behaviors, and attentional problems and hyperactivity. Physical characteristics of children with Fragile X include a long face, large ears, flat feet, and extremely flexible joints, especially fingers. Boys are affected more often than girls, and are more likely to have mental retardation rather than milder learning impairment. Both boys and girls are likely to have emotional and behavioral problems. Recent studies suggest that Fragile X affects about one in 2,000 males and one in 4,000 females of all races and ethnic groups.
Infections during pregnancy can cause a variety of birth defects. Some examples are:
- Congenital rubella (the infection posing the highest risk for fetal damage) syndrome is characterized by vision and/or hearing loss, heart defects, mental retardation, and cerebral palsy.
- Toxoplasmosis infection of the mother can result in eye infections that threaten vision, hearing loss, learning disabilities, enlarged liver or spleen, mental retardation, and cerebral palsy in the infant.
- Genital herpes virus infection of the mother can cause brain damage, cerebral palsy, vision or hearing impairment, and death of the baby if the virus is transmitted to the infant before or during the birth.
- Fifth disease can cause a dangerous form of anemia, heart failure, and, in some cases, fetal death,
- Congenital varicella syndrome, which is caused by chicken pox, can lead to scars, defects of muscle and bone, malformed and paralyzed limbs, a smaller-than-normal head, blindness, seizures, and mental retardation. This is an uncommon occurrence in pregnant women who become infected with chicken pox.
Routine prenatal screening can do two major things: it can help determine if the mother has an infection or other condition that is dangerous to the fetus, and it can help determine if the fetus has certain birth defects. The list of defects that may be detected through prenatal screening includes:
- neural tube defects (spina bifida, anencephaly)
- Down syndrome
- other chromosome abnormalities
- inherited metabolic disorders
- congenital heart defects
- gastrointestinal and kidney malformations
- cleft lip or palate
- certain birth defects of the limbs
- congenital tumors
Informations obtained from National Institute of Health.