A birth defect is a problem that happens while a baby is developing in the mother's body. Most birth defects happen during the first 3 months of pregnancy. One out of every 33 babies in the United States is born with a birth defect.

A birth defect may affect how the body looks, works or both. Some birth defects like cleft lip or neural tube defects are structural problems that can be easy to see. To find others, like heart defects, doctors use special tests. Birth defects can vary from mild to severe. Some result from exposures to medicines or chemicals. For example, alcohol abuse can cause fetal alcohol syndrome. Infections during pregnancy can also result in birth defects. For most birth defects, the cause is unknown.

Some birth defects can be fatal. Babies with birth defects may need surgery or other medical treatments. Today, doctors can diagnose many birth defects in the womb. This enables them to treat or even correct some problems before the baby is born.

What Are 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%.

Other causes of birth defects include alcohol abuse by the mother and Rh disease, which can occur when the mother's and baby's Rh factors (the "positive" or "negative" part of a person's blood type) are different. Although a few medications can cause problems, of the 200 most commonly prescribed drugs, none is associated with a significant risk of birth defects.

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 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.