What Are Birth Defects?

While still in the womb, some babies have problems with how their organs and body parts form, how they work, or how their bodies turn food into energy. These health problems are called birth defects.

There are more than 4,000 different kinds of birth defects, ranging from minor ones that need no treatment to serious ones that cause disabilities or require medical or surgical treatment.

What Are the Types of Birth Defects?

If a baby is born with a part of the body that is missing or malformed, it is called a structural birth defect. Heart defects are the most common type of structural defect. Others include spina bifida, cleft palate, clubfoot, and congenital dislocated hip.

When there is a problem with a baby's body chemistry, it is called a metabolic birth defect. Metabolic defects prevent the body from properly breaking down food to create energy. Examples of 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.

For people who want to become parents, it's important to know that some birth defects can be prevented. During a woman's pregnancy, taking folic acid and getting enough iodine in the diet can help prevent some types of birth defects. But it's also important to realize that most babies born with birth defects are born to two healthy parents with no obvious health problems or risk factors.

What Causes Birth Defects?

In most cases, doctors don't know what caused a baby's birth defect. When the cause is known, it might be environmental (such as a baby's exposure to chemicals or viruses while in the womb), a problem with genes, or a combination of these things.

Environmental Causes

If a mother has certain infections (such as toxoplasmosis) during pregnancy, her baby can have a birth defect. Other conditions that cause defects include rubella and chickenpox (varicella). Fortunately, many people get vaccinated against these diseases so these infections are rare.

Also, alcohol abuse by the mother may cause fetal alcohol syndrome, and certain medicines taken by the mother can cause birth defects. (Doctors try to avoid harmful medicines during pregnancy, so a pregnant woman should tell any doctor she consults that she's expecting.)

Genetic Causes

Every cell in the body has chromosomes containing genes that determine a person's unique characteristics. During conception, a child inherits one of each pair of chromosomes (and one of each pair of the genes they contain) from each parent. An error during this process can cause a baby to be born with too few or too many chromosomes, or with a damaged chromosome.

One well-known birth defect caused by a chromosome problem is Down syndrome. A baby develops Down syndrome after getting one extra chromosome. Other genetic defects happen when both parents pass along a faulty gene for the same disease.

A disease or defect also can happen when only one parent passes along the gene for that disease. This includes birth defects such as achondroplasia (a form of dwarfism) and Marfan syndrome.

Finally, some boys inherit disorders from genes passed onto them only by their mothers. These defects, which include conditions like hemophilia and color blindness, are called X-linked because the genes are carried on the X chromosome. 

How Are Birth Defects Diagnosed?

Many birth defects are diagnosed even before a baby is born through prenatal tests. Prenatal tests also can help determine if a mother has an infection or other condition that is dangerous for the fetus. Knowing about a baby's health problems ahead of time can help parents and doctors plan for the future.

It's important to remember that screening identifies only the possibility that a baby has a defect. Some women give birth to a healthy baby after a screening test shows that a defect may be present. If you're pregnant, talk to your doctor about any tests he or she thinks you should have.

Other birth defects are found during routine newborn screenings. With parents' permission, babies are tested after birth to screen for certain birth defects that need to be treated. In the U.S., exactly what a baby is tested for varies from state to state, although all states screen for phenylketonuria (PKU), congenital hypothyroidism, sickle cell disease, and about 30 other conditions. Parents should ask health care providers or the hospital nursery which tests their state does.

Parents who have concerns about another specific birth defect might be able to have their baby tested for it. They should talk to their health care provider about it before the baby is born.

Can Birth Defects Be Prevented?

Many birth defects can't be prevented, but a woman can do some things before and during pregnancy to help lower the chances of having a baby with a birth defect.

Before pregnancy, women should:

  • make sure their vaccinations are up to date
  • make sure they don't have any sexually transmitted diseases (STDs)
  • get the daily recommended dose of folic acid before trying to conceive
  • avoid unnecessary medicines and talk to their doctor about medicines they are taking

If there's a family history of birth defects or a woman is part of a high-risk group, she should consider meeting with a genetic counselor to determine her baby's risk.

During pregnancy, it's important to take prenatal vitamins and eat a healthy diet in addition to taking the following precautions:

  • don't smoke and avoid secondhand smoke
  • don't drink alcohol
  • avoid all illicit drugs
  • get exercise and plenty of rest
  • get early and regular prenatal care

By following these pregnancy precautions, women can help reduce their babies' risk of birth defects.

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Note: All information is for educational purposes only. For specific medical advice, diagnoses, and treatment, consult your doctor. © 1995-2020 KidsHealth®. All rights reserved. Images provided by The Nemours Foundation, iStock, Getty Images, Veer, Shutterstock, and Clipart.com.

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