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Overview of Birth Defects

By Nina N. Powell-Hamilton, MD, Clinical Assistant Professor of Pediatrics ;Medical Geneticist , Sidney Kimmel Medical College at Thomas Jefferson University;Nemours/Alfred I. duPont Hospital for Children

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 (miscarriage).

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 Common Birth Defects

Major System

Birth Defect

What Happens

Treatment

Heart

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

Digestive tract

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

Surgery to close the defect

Musculoskeletal

Limb may not form or may be "amputated" in the womb

Artificial limb and therapy to help the child adapt and function

Prune-belly syndrome

Missing layers of abdominal muscles, causing the abdomen to bulge

Urinary system defects often develop

Surgery if a urinary system defect blocks urine flow

Neurologic

Porencephaly

Brain tissue missing and replaced with fluid-filled sacs

No treatment available

Ventricular shunt may decrease pressure

Hydranencephaly

Severe porencephaly with little remaining brain tissue

No treatment available

Genital

Vanishing testes (bilateral anorchia or testicular regression) and an unusually small penis (micropenis)

Both testes absent at birth, resulting in a micropenis

Supplements of the male hormone testosterone beginning before puberty

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.

Nutrition

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 chromosome abnormalities or gene mutations in the child. Birth defects caused by genetic factors often include more than just an obvious malformation of a single body part.

Infections

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

  • 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 (see Prenatal Diagnostic Testing). The chance is higher for women who have the following risk factors:

  • Are older than 35 years of 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. 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 (see Screening of the Pregnant Woman : 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 testing (NIPT). NIPT 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 be 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), are done based on the results of the physical examination and screening tests.

Treatment

  • Sometimes surgery or drugs

Abnormal chromosomes or genes cannot be corrected.

However, 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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