What We Know



Birth Defects: What We Know

last updated in 2004

What Are Birth Defects?

People define “birth defects” in different ways and recognize different types of birth defects.

  • Structural defects present at birth that result in a physical disability or death are birth defects by anyone’s definition. Structural birth defects affect the formation of parts of the body, and include cleft palate (a split in the roof of the mouth), spina bifida (a defect in the neural tube that allows part of the spinal cord to protrude from the spine), certain heart defects, and missing arms or legs, to give just a few examples.
  • Functional defects are considered by some, but not all, to be birth defects. They may be considered birth defects if they result from untoward events that occur during fetal development. Functional defects can affect the nervous system, immune system, reproductive system, endocrine system, or other systems of the body, and may not become apparent for months or years.These may include, for example, mental retardation, autism, attention deficit hyperactivity disorder (ADHD), some types of blindness and deafness, some immunological problems, and some degenerative disorders.
  • Metabolic defects, or abnormalities in body chemistry, are considered by the March of Dimes organization to be birth defects (specifically, functional birth defects). Some metabolic defects are recessive genetic diseases and some people do not consider them to be birth defects. One example is Tay-Sachs disease. Babies with Tay-Sachs lack an enzyme needed to break down certain fatty substances in brain cells. These substances build up and destroy brain cells, resulting in blindness, paralysis and death by age five. Another example is phenylketonuria (PKU). Babies with PKU cannot process an amino acid (a building block of proteins) , which then builds up in blood and causes brain damage. PKU is routinely detected with newborn screening tests, so affected babies can be placed on a special diet that prevents mental retardation.
  • Non-disabling defects are considered by some, but not all, to be birth defects. These are abnormalities that do not necessarily result in a disability, although they may be unwanted or cosmetically disfiguring, such as an extra finger.

These differences in definition add to the challenges faced by those who study the causes of birth defects and how they have changed over time.

How Serious are Birth Defects?

Major birth defects are the leading cause of infant mortality in the United States.

Birth defects that most often cause infant death include heart defects, respiratory defects, nervous system defects, and multiple abnormalities. 

Birth defects also are a major cause of miscarriage and fetal death. Nearly half of all pregnancies today result in the loss of the embryo or fetus, often very early in pregnancy. Many of these losses are due to a problem that would result in a birth defect.

According to one estimate, it costs about $8 billion over a lifetime for children born each year in the U.S. with one or more of 18 of the most significant major birth defects, including cerebral palsy.

Birth defects can vary in their seriousness but many pose profound physical, mental, social, and economic hardships on the individual and their families.

How Common are Birth Defects?

An exact answer is not known, since data on birth defects are not collected in a uniform and complete way. For example, 1/3 of the states in the U.S. lack any system for tracking birth defects. Birth defects that are not detected until months or years after birth are often missed by existing systems for tracking birth defects. And except for some parts of California and Atlanta, Georgia, no states track functional defects such as mental retardation or cerebral palsy.

About 3.5% of all babies have structural birth defects according to hospital discharge records. This estimate is thought to be an underestimate of structural birth defects since it does not include defects that are detected later in life. It also does not include functional birth defects. One large study that followed children for seven years found that 16% of children had structural birth defects. Half of these were major birth defects and half were less serious.

What Causes Birth Defects?

We know the causes for some birth defects, but the causes of most birth defects are unknown. We do know that genetics, environmental factors, and nutritional, infectious, and other health-related factors during pregnancy play a role, but how big or small a role each plays in causing birth defects overall is the matter of intense research and debate.

A growing number of experts believe that most birth defects result from multiple factors. By this interpretation, inherited genes may predispose someone to a birth defect, but one or more environmental factors also must be present to result in a birth defect. This is called a gene-environment interaction. For example, women with a certain form of a gene that is involved in the development of the mouth are more susceptible to having a child with cleft lip or cleft palate, if they also smoke. If they don’t smoke, their risk is much lower. And the risk is also much lower for women who smoke but who don’t have that particular gene.

Genetics: We know that some birth defects result from one or more genes or chromosomes that are inherited from one or both parents. The gene or chromosome may be damaged in the parent, either because he or she inherited it that way from their parents, or because it was damaged sometime during their lifetime by an environmental agent that caused a mutation or other abnormality. For example, a single faulty gene carried by a parent can result in a type of dwarfism called achondroplasia in the child. Another example is children with Down syndrome who have an extra chromosome.

Environmental factors linked to birth defects include radiation, alcohol, drugs (such as an anti-seizure medication called Dilantin and medications called retinoids used to treat severe acne), smoking, lead, mercury, air pollution, solvents, pesticides, dioxins, and PCBs. The evidence linking environmental factors and birth defects varies for different environmental factors. In some cases the evidence is very strong, as it is for fetal alcohol syndrome, a set of birth defects caused by the mother drinking alcoholic beverages during pregnancy. In other cases the evidence may be less consistent or weaker, and may implicate but not prove that a particular agent causes birth defects. Both the dose and the timing of the exposure are critical to determining whether a particular environmental agent will actually cause birth defects.

Infections, nutritional deficiencies, and other health factors during pregnancy have been linked to some birth defects. For example, not getting enough of the B vitamin called folic acid during pregnancy can lead to birth defects of the neural tube, such as spina bifida, in the baby. Infection with rubella (German measles) in the first trimester of pregnancy can result in multiple birth defects (congenital rubella syndrome). The parasitic disease toxoplasmosis, transmitted through raw meat and cat feces can cause malformations of the brain, liver and spleen if a fetus becomes infected in the first trimester. Syphilis (if untreated) and a herpes virus called cytomegalovirus can cause birth defects in the infants of women who have these infectious diseases. Birth defects are also more frequent in children of mothers who have diabetes or thyroid disorders.

What role do other environmental agents play in causing specific birth defects?

Below are a few examples of links between environmental agents and specific birth defects:

  • Endocrine disrupting chemicals cause birth defects in laboratory animals and are likely to contribute to birth defects in people. For example, chemicals that disrupt estrogen are suspected of causing an increase in the incidence of hypospadias, a birth defect found in boys in which the urinary tract opening is not located at the tip of the penis as it is normally. Although genetic factors play an important role in hypospadias, they cannot explain the doubling in the rate of hypospadias between 1970-1993, and neither can changes in reporting or tracking systems.
  • Hazardous Waste Sites: Women who live within ¼ mile of a Superfund site during the first 3 months of pregnancy may have a greater risk of having a baby with serious heart and neural tube defects, according to a California Birth Defects Monitoring Program study. While this study did not measure specific exposures, it does add to other evidence linking hazardous waste sites with these specific birth defects.
  • Pesticides: A number of studies link pesticides with certain types of birth defects, including birth defects that result in death of the fetus. One study suggests that herbicides used on wheat may cause birth defects.
  • Plastic chemicals: A recent study in laboratory animals has linked very low doses of the plastic chemical bisphenol A (BPA) to aneuploidy, an error in cell division that causes spontaneous miscarriages and birth defects in people, include Down’s syndrome. More work is needed to determine whether and to what degree BPA may cause the same results in people.
  • Solvents: A number of studies link solvents with certain types of birth defects. For example, several lines of evidence link solvents and gastroschisis, a life-threatening condition where the intestines protrude through a hole in the abdomen.

A number of these and other environmental factors are linked to functional alterations in the developing brain.

Why isn’t more known?

Here are some of the reasons:

  • Most commonly encountered chemicals (other than pesticides and drugs) are not tested for their ability to cause birth defects.
  • Tests required for new pesticides and drugs are often not sensitive enough to identify less obvious birth defects, including many functional defects.
  • Laboratory animals used in tests for birth defects are genetically very similar to each other and have carefully controlled diets and exposures. These tests can not always predict what will happen in the real world, which is far more complex and diverse, and where there are many different people with different genetic backgrounds, diets, lifestyles, health conditions, and exposures, including multiple exposures.
  • Studies in people are often unable to identify and/or estimate accurately exposures that occurred months or years previously.
  • The ability of an agent to cause birth defects often depends on when exposure to that agent occurs. If researchers don’t study exposures at that point in time, they will likely miss its ability to cause birth defects. For example, children exposed to the drug thalidomide during the third to sixth week of pregnancy often suffered limb deformities, while children exposed later either had no effects or different effects.
  • Certain birth defects are rare, making it hard to design studies powerful enough to reliably detect them.
  • Interactions between multiple factors (environmental, genetic, health) make it hard to pinpoint the contribution of one factor.
  • There is no comprehensive national system for monitoring or reporting birth defects.

Where does that leave us now?

  • We know that the developing embryo and fetus is extraordinarily and uniquely vulnerable to environmental exposures. These exposures can result not only in structural birth defects, but also can impact the function of the nervous, immune, reproductive, and other systems of the body. Some of these impacts are not apparent until years or decades after birth.
  • A woman accumulates contaminants over a lifetime of exposure prior to pregnancy. During pregnancy she can transfer large quantities of contaminants to the fetus, just at the time in the life of her baby when it is most vulnerable.
  • A “better safe than sorry” approach based on the precautionary principle should be taken when dealing with chemicals that may cause birth defects. While such an approach is most effectively taken by government and industry, there fortunately are manycommon sense steps that individuals and families can take as well.

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