Birth Defects and Brain Development

Birth defects of the brain are a group of disorders that result from illness or injury to the brain and central nervous system of a developing fetus * . Birth defects can be inherited from a parent or acquired through the mother's contact with environmental factors such as drugs or infections. Birth defects of the brain result in problems that can range from mild to severe. They can affect one part or many parts of the central nervous system.


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Cerebral palsy

Congenital disorders

Cytomegalovirus (CMV)

Fetal alcohol syndrome

Genetic disorders

Phenylketonuria (PKU)


Spina bifida



The Developing Fetus

Starting at conception, the developing embryo * is susceptible to many factors that affect development. During the first twelve weeks of development, the cells of the embryo and fetus are rapidly dividing and becoming the infant's muscles, bones, and organs. Exposure to disease or toxins * may disrupt the process of development and can result in birth defects and developmental problems in the infant.

* fetus (FEE-tus) in humans, is the unborn offspring in the period after it is an embryo, from 9 weeks after fertilization until birth.

* embryo (EM-bree-o) in humans, the developing organism from the end of the second week after fertilization to the end of the eighth week,

* toxins (TOK-sinz) are poisonous substances.

Since rapid growth and organ formation occur during the first 12 weeks of development, that is when the embryo and fetus are most susceptible to injury or insult. The neural (NU-ral) tube, which develops into the spinal cord and the brain, begins to develop about 10 days after conception and continues to develop throughout the pregnancy. Because the mother and the developing fetus are connected, many things that the mother is exposed to or takes into her body can be passed to the fetus. If the mother becomes ill, takes medications, is exposed to toxins, or takes any drugs, the fetus may be affected. How severe the developmental problem that results will be depends on many factors including the type of illness or type or amount of toxin involved.

The developing fetus must also receive enough oxygen. Low levels of oxygen reaching the fetus result in what is called intrauterine hypoxia (hy-POX-ee-a). Because cells need oxygen to survive and grow, hypoxia can cause developmental problems in the central nervous system, especially in the early stages of development. Anything that disrupts development at this young age can cause effects that last throughout childhood and adulthood. These types of central nervous system defects are not generally correctable after birth.

When the developing brain is exposed to a toxin it is difficult to predict exactly which parts of the brain will be affected or how severely the brain will be damaged. Disorders of intrauterine brain and central nervous system development lead to congenital (con-JEN-it-al) disorders or birth defects. It is important to note that the cause of the majority of birth defects of the central nervous system is unknown or uncertain, that is, no specific genetic abnormality, or toxin, drug, infection, or other environmental factor can be identified.

What follows are descriptions of some of the most common types of central nervous system birth defects and some of the agents that may cause them.


Anencephaly (an-en-SEF-a-lee) is a defect in the neural tube, which is the part of the developing fetus that forms the spinal cord and brain. During early development, the neural tube fails to close and results in a missing or malformed brain or spinal cord. A fetus with anencephaly is born with no brain or only the very basic parts of the brain that control processes like breathing. Anencephaly is always a fatal condition and the infant may be stillborn * or die within days or weeks of birth.

The exact cause of this disorder is still not known. Some studies have suggested that a problem with the nutrition of the mother may be an important cause of this disorder. Lower than normal levels of the B vitamin folic acid in the mother may place the developing fetus at risk for anencephaly.

* stillborn means a baby who is not a live at birth.

Spina Bifida

Spina bifida (SPY-na BIF-i-da) or myelomeningocele (MY-eh-lo-me-NING-oh-seel) is a congenital disorder of neural tube development where there is an opening in the spinal cord and the spinal column (backbone) of the developing fetus. Spina bifida is one of the most common birth defects that involve the nervous system and it may affect as many as 1 in 800 infants. As with anencephaly, deficiency of folic acid in the mother appears to be an important factor in the development of spina bifida. Because the spinal cord is not enclosed in the backbone, the infant has a high risk for meningitis, a serious infection of the spinal fluid, brain, and the lining covering the brain and spinal cord. Generally the affected person has a loss of body functions below the level of the opening in the

This infant was born with spina bifida. During fetal development, the cells of the neural tube differentiate to form the brain and spinal cord. Spina bifida ("split spine") is a neural tube defect in which the spinal column does not close completely before birth. Custom Medical Stock Photos
This infant was born with spina bifida. During fetal development, the cells of the neural tube differentiate to form the brain and spinal cord. Spina bifida ("split spine") is a neural tube defect in which the spinal column does not close completely before birth.
Custom Medical Stock Photos
spinal cord. While surgery at birth will repair the opening, it will not change the loss of function. Children with spina bifida often do not walk unassisted and frequently have learning disabilities. Despite this, many children with spina bifida grow up to live productive lives.

Embryo Toxins

Embryo toxins are any substances that negatively affect or injure the embryo. In general, embryo toxins are called teratogens (tare-AT-o-jenz). Teratogens are substances that cause birth defects when the mother is exposed to them. Teratogens may include over-the-counter or prescription medicines, illegal drugs, common household chemicals, industrial waste, alcohol, or tobacco. It is important that the mother avoid teratogens while pregnant, especially during the first twelve weeks of pregnancy when the organs of the embryo and fetus are forming. Pregnant women or those seeking to become pregnant should consult with their physicians about which medications they can and cannot take.


Alcohol is a teratogen that should be avoided during pregnancy. Mothers who drink during pregnancy put their child at risk for developing fetal alcohol syndrome (FAS). FAS can result in growth problems as well as mental and other physical congenital defects. Alcohol is a toxin that affects the developing central nervous system of the fetus. Children with fetal alcohol syndrome often show slow physical growth, delayed mental development that can be mild to severe, facial abnormalities, heart defects, and abnormalities of the skeleton. These children usually have mental retardation and frequently have behavior problems and hyperactivity.


Nicotine from cigarette smoke is another teratogen that should be avoided during pregnancy. Nicotine from the mother's blood-stream is passed to the developing fetus. The nicotine affects the developing fetus and can contribute to a wide variety of problems, including low birth weight, birth defects, and sudden infant death syndrome (SIDS). Another problem caused by smoking is the lowering of blood oxygen content in the mother. When the mother's blood oxygen falls, the fetus also receives less oxygen, a condition called fetal hypoxia. This may contribute to the damaging effects of maternal smoking on the fetus.

A doctor examines a child with spina bifida at a clinic near the Texas-Mexico border. The child's disorder is believed to have been caused by pollution in the area. Annie Griffiths Belt/Corbis
A doctor examines a child with spina bifida at a clinic near the Texas-Mexico border. The child's disorder is believed to have been caused by pollution in the area.
Annie Griffiths Belt/Corbis

Medications and Street Drugs

Some drugs can act as teratogens and should be avoided during pregnancy. Illegal drugs such as amphetamines, cocaine, and heroin may affect the developing central nervous system of the fetus. In addition, these drugs may lead to other birth defects as well as addiction problems. Illegal drugs are not the only drugs that can cause problems. Prescription and over-the-counter medications can also affect the fetus. Pregnant mothers should consult with a physician about which medications are safe to take and which should be avoided.

Environmental hazards

Exposure to household chemicals or other toxins in the environment may also contribute to impairment of the central nervous system of a fetus. In 1994 and 1995, a greater than expected number of infants along the Rio Grande in Texas and Mexico were born with anencephaly. While the exact cause was not found, it is suspected that pollution in the environment was involved. It is probably wise for pregnant women to avoid pesticides (such as weed killers) and insecticides. Some household cleaning chemicals contain substances that are teratogens and these should be avoided also.

Embryo Infections

Toxins in the environment are not the only things that affect the developing central nervous system. A number of infectious diseases may also affect the developing embryo. Some of these diseases may or may not affect the mother but can be passed to the fetus. Many of these diseases are caused by viruses, which are easily passed between the mother and fetus. Some infections that can have major effects on the fetus are:


Cytomegalovirus (sy-to-MEG-a-lo-vy-rus; CMV) is a common virus that infects most people at some point in their lives. In some cases, the mother may pass the virus to the fetus through the placenta. In adults, CMV infections usually do not cause any significant symptoms. For the developing fetus, however, the infection can damage the brain. Children who had fetal CMV infections may have stunted growth, microcephaly (my-kro-SEF-a-lee; abnormally small brain and head size), and a number of other physical problems. Some of the symptoms may be treatable, but the central nervous system injury is permanent.


Syphilis is a sexually transmitted bacterial disease that can be passed from mother to fetus. The risk of this occurring is 60 to 80 percent if the mother is infected and does not receive treatment. Early in the pregnancy, the developing central nervous system of the fetus may be affected by the infection. At birth, some of these infants have meningitis (inflammation of the brain, spinal fluid, and lining covering the brain) or hydrocephalus (hy-dro-SEF-a-lus; excessive fluid within and surrounding the brain), and may have mental retardation as they grow older. Infants with the infection are treated with antibiotics to control the infection. This treatment may prevent additional damage to the nervous system but it does not reverse damage done to the developing brain before birth.

Rubella (German measles)

Those who have received rubella vaccine are protected against infection. In the early stages of pregnancy, an unvaccinated, exposed mother can pass rubella to the developing fetus. The developing brain and central nervous system of the infant may be affected causing cerebral palsy and mental retardation, among other problems. There is no treatment that can reverse the brain damage rubella infection causes before birth.


Toxoplasmosis (tok-so-plaz-MO-sis) is an infection caused by a parasite that is commonly found in undercooked meat and in cat stools. In most cases, there are no symptoms of the infection in healthy adults. However, the infection can be passed from the mother to the developing fetus. About 85 percent of infants born infected with toxoplasmosis show no symptoms at birth. Later, however, these children may develop learning disabilities, movement problems, mental retardation, and vision loss. Pregnant women should make sure that meat is cooked completely and they should avoid changing the cat litter box.

Metabolic Disorders

Metabolic (met-a-BOLL-ik) disorders are problems in the chemical reactions in the body involved with producing energy from food, and supporting the growth and function of the body's cells and tissues. When metabolic disorders occur during pregnancy or even during childhood, they may affect the developing central nervous system.

In conditions that affect the development of the central nervous system after birth, treatment for the condition may prevent problems such as mental retardation and other central nervous system problems. Two of the most common metabolic conditions that affect children are phenylketonuria (PKU) and hypothyroidism.

Phenylketonuria (PKU)

Phenylketonuria (fen-il-kee-toe-NYOOR-ee-a) is an inherited genetic disorder that afflicts about 1 in 15,000 children. Children with this disorder are unable to chemically handle phenylalanine, an essential amino acid. If affected infants are not placed on a special diet soon after birth, they may develop seizures and permanent mental retardation. The diet for PKU restricts foods that contain phenylalanine.


Hypothyroidism is another metabolic condition that can affect the development of the brain and central nervous system. Hypothyroidism is a disease of the thyroid gland where there is an un-derproduction of thyroid hormones that are necessary for many body functions, including brain development in infants and children. Congenital hypothyroidism occurs in about 1 in 4,000 children, usually caused by failure of the thyroid gland to develop normally in the fetus. If it is left untreated, the infant's brain will not develop properly and the child will have slow physical growth and impaired mental abilities. Treatment with thyroid hormone (in pill form) started in the first few weeks of life will usually prevent developmental problems in the child.

See also
Cerebral Palsy
Fetal Alcohol Syndrome
Genetics and Behavior
Mental Retardation

Living with Birth Defects

Since currently there is no way to repair brain damage that occurs during early development, it is important to do what is possible to prevent these conditions. During pregnancy, the mother should take care of herself. She should avoid smoking, illegal drugs, and alcohol, taking medications without a doctor's approval, eating undercooked meat, cleaning the cat litter box, and exposure to toxic chemicals. She should eat a balanced diet and make sure she gets enough folic acid. That's a lot of things to think about when a woman is about to have a baby! Making frequent visits to the doctor when pregnant for advice on these issues is a good way to help give baby the best start possible.

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