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Overview of Birth Defects


Nina N. Powell-Hamilton

, MD, Sidney Kimmel Medical College at Thomas Jefferson University

Last full review/revision Jul 2019| Content last modified Jul 2019
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Birth defects, also called congenital anomalies, are physical abnormalities that occur before a baby is born. They are usually obvious within the first year of life.

  • The cause of many birth defects is unknown, but infections, genetics, and certain environmental factors increase the risk.

  • Before the baby is born, the diagnosis may be based on the mother's risk factors, the results of an ultrasound, and sometimes blood tests, amniocentesis, or chorionic villus sampling.

  • After the baby is born, the diagnosis may be based on a physical examination, imaging tests, and blood tests.

  • Some birth defects can be prevented by maintaining good nutrition while pregnant and avoiding alcohol, radiation, and certain drugs.

  • Some birth defects can be corrected with surgery or managed with drugs.

Birth defects can involve any part of any organ in the body, including the following:

Some birth defects are more common than others.

Birth defects are the leading cause of death in infants in the United States, and some cause the death of the fetus, causing miscarriage or stillbirth.

A birth defect is evident in about 7.5% of all children by age 5 years, although many of these are minor. Major birth defects are evident in about 3 to 4% of newborns.

Several birth defects can occur together in the same infant.


Some Birth Defects

Organ System

Birth Defect

What Happens


Hypoplastic left heart syndrome

Underdevelopment of the left ventricle, leading to inability to pump blood to the body

Separate operations to rebuild the left ventricle or a heart transplant

A hole in, or weakening of, abdominal muscles, allowing internal abdominal organs to spill out (protrude) through the defect

Surgery to close the defect

The foot and ankle are twisted out of shape or position

Immobilization with a cast and physical therapy to stretch the foot and ankle

Sometimes surgery


Brain tissue missing and replaced with fluid-filled sacs

No treatment available

Ventricular shunt may decrease pressure


Severe porencephaly with little remaining brain tissue

No treatment available

A testis (testicle) remains in the abdomen or groin instead of descending into the scrotum before birth

About half will descend into the scrotum without treatment by 4 to 6 months

Surgery to move the testis into place

Causes of and Risk Factors for Birth Defects

It is not surprising that birth defects are fairly common, considering the complexities involved in the development of a single fertilized egg into the millions of specialized cells that constitute a human being. Although the cause of most birth defects is unknown, certain genetic and environmental factors increase the chance of birth defects developing. These factors include exposure to radiation, certain drugs (see Table: Some Drugs That Can Cause Problems During Pregnancy*), alcohol, nutritional deficiencies, certain infections in the mother, and genetic disorders.

Some risks are avoidable. Others occur no matter how strictly a pregnant woman adheres to healthful living practices. Many birth defects develop before a woman knows she is pregnant.

Exposure to harmful substances (teratogens)

A teratogen is any substance that can cause or increase the chance of a birth defect. Teratogens include

  • Radiation (including x-rays)

  • Certain drugs

  • Toxins (including alcohol)

Most pregnant women who are exposed to teratogens have newborns without abnormalities. Whether a birth defect occurs depends on when, how much, and how long the pregnant woman was exposed to the teratogen (see Exposures During Pregnancy).

Exposure to a teratogen most commonly affects the fetal organ that is developing most rapidly at the time of exposure. For example, exposure to a teratogen during the time that certain parts of the brain are developing is more likely to cause a defect in those areas than exposure before or after this critical period.


Keeping a fetus healthy requires maintaining a nutritious diet. For example, insufficient folic acid (folate) in the diet increases the chance that a fetus will develop spina bifida or other abnormalities of the brain or spinal cord known as neural tube defects. Cleft lip (a separation of the upper lip) or cleft palate (a split in the roof of the mouth) is also more likely to develop.

Obesity in the mother also increases the risk of a neural tube defect.

Genetic and chromosomal factors

Chromosomes and genes may be abnormal. These abnormalities may be inherited from the parents, who may either be affected by the abnormalities or who may be carriers of the genes that cause the abnormalities (see Overview of Chromosome and Gene Disorders). Carriers are people who have an abnormal gene for a disorder but who do not have any symptoms of the disorder.

However, many birth defects are caused by new chromosome abnormalities or gene mutations that arise in the child and were not inherited from the parents.

Birth defects caused by genetic factors often include more than just an obvious malformation of a single body part.


Certain infections in pregnant women can cause birth defects (see Infections During Pregnancy). Whether an infection causes a birth defect depends on the age of the fetus at the time of the exposure to the infection.

The infections that most often cause birth defects are

A woman can have one of these infections and not know it because some of these infections can cause few or no symptoms in adults.

Diagnosis of Birth Defects

  • Before birth, ultrasonography and sometimes magnetic resonance imaging, blood tests, amniocentesis, or chorionic villus sampling

  • After birth, physical examination, ultrasonography, computed tomography, magnetic resonance imaging, and blood tests

Before birth

Before birth, doctors assess whether a woman is at increased risk of having a baby with a birth defect (Prenatal Diagnostic Testing). The chance is higher for women who have the following risk factors:

  • Older age

  • Have had frequent miscarriages

  • Have had other children with chromosome abnormalities or birth defects or who died for unknown reasons

These women may need monitoring and special tests to find out whether their baby is developing normally.

Increasingly, birth defects are being diagnosed before the baby is born.

Ultrasonography of the fetus is commonly done during pregnancy. Magnetic resonance imaging (MRI) of the fetus may also be done when indicated. These imaging tests can often detect specific birth defects.

Sometimes blood tests can also help. For example, a high level of alpha-fetoprotein in the mother's blood may indicate a defect of the brain or spinal cord or certain other organs (Second-Trimester Screening). More recently, doctors are using a test called cell-free fetal DNA analysis. In this test, a sample of a pregnant woman's blood is analyzed to determine whether her fetus has certain genetic disorders. This test is based on the fact that the mother's blood contains a very small amount of DNA (genetic material) from the fetus. This test is called noninvasive prenatal screening (NIPS). NIPS can be used to detect an increased risk of trisomy 21 (Down syndrome), trisomy 13, or trisomy 18 and certain other chromosome disorders. Doctors usually do further testing when an increased risk of a gene abnormality is detected.

Amniocentesis (removing fluid from around the fetus) or chorionic villus sampling (removing tissue from the sac around the developing fetus) may help confirm a suspected diagnosis. Genetic testing can be done on the samples taken during these tests.

After birth

After birth, the doctor does a physical examination of the newborn. During this examination, the doctor examines the newborn's skin, head and neck, heart and lungs, and abdomen and genitals and assesses the newborn's nervous system and reflexes. Some newborns have a physical appearance that suggests a certain disorder.

In the United States, most newborns undergo routine screening blood tests to detect a number of metabolic disorders.

Imaging tests, such as ultrasonography, computed tomography (CT), and magnetic resonance imaging (MRI), may be done based on the results of the physical examination and screening tests.

Treatment of Birth Defects

  • Sometimes surgery or drugs

Abnormal chromosomes or genes cannot be corrected.

Surgery can correct or help some birth defects. Drugs and surgery can be used to manage the symptoms caused by other defects.

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