Medical Genetics
Overview of Birth Defects
What is a birth defect?
A "birth defect" is a health problem or physical change, which is present in a baby at the time he/she is born. Birth defects may be very mild, where the baby looks and acts like any other baby, or birth defects may be very severe, where you can immediately tell there is a health problem present. Some of the severe birth defects can be life threatening, where a baby may only live a few months, or may die at a young age (in their teens, for example).
Birth defects are also called "congenital anomalies" or "congenital abnormalities." The word "congenital" means "present at birth." The words "anomalies" and "abnormalities" mean that there is a problem present in a baby.
How frequently do birth defects occur?
Most babies are born healthy. In fact, 96 to 97 out of every 100 babies born is born healthy. About three or four out of every 100 babies born, however, has some type of birth defect.
What causes birth defects to occur?
There are many reasons why birth defects happen. Most occur due to environmental and genetic factors. About 40 percent of all birth defects have a known cause. The remaining 60 percent of birth defects do not have a known cause. You may find it surprising that scientists and physicians have not determined the cause for all birth defects. This is why there is a lot of research into the causes of birth defects, to understand more about why they happen and how to prevent them.
Who is affected by birth defects?
Birth defects have been present in babies from all over the world, in families of all nationalities and backgrounds. Anytime a couple becomes pregnant, there is a chance that their baby will have a birth defect. As mentioned above, this chance is three to four out of 100, or 3 to 4 percent. This means that there is a 96 to 97 percent chance with each pregnancy, for a baby to be born normal and healthy. The "3 to 4 percent" number is sometimes called the background rate for birth defects, or the population risk for birth defects.
In a family where birth defects are already present, the chance for a couple to have a child with a birth defect may be higher than the background rate of 3 to 4 percent.
What are the genetic and environmental causes of birth defects?
When a baby is born with a birth defect, the first question usually asked by the parents is "how did this happen?" Sometimes, this question cannot be answered. This can be very upsetting for parents because it is normal to search for and desire an answer as to why your baby has a health problem. For about 40 percent of birth defects, however, there is a known cause, which has to do with either genetic or environmental factors, or a combination of the two. Here is some general information and terms related to the different causes of birth defects:
Inheritance
Inheritance is a word used to describe a trait given to you or "passed on" to you from one of your parents. Examples of inherited traits would be your eye color or blood type.Chromosome abnormalities
Chromosomes are stick-like structures in the center of each cell (called the nucleus) that contain your genes. Problems involving chromosome material cause about 6 percent of all birth defects.Single gene defects
A defect in a gene is seen in about 7.5 percent of all birth defects. Genes are what determine your traits. Sometimes, a child can inherit not only those genes responsible for their normal traits such as the color of their eyes, but also disease causing genes that result in a birth defect.Multifactorial inheritance
Multifactorial inheritance means that "many factors" (multifactorial) are involved in causing a birth defect. The factors are usually both genetic and environmental. About 20 percent of all birth defects are the result of multifactorial inheritance.Teratogens
A teratogen is an agent, which can cause a birth defect. It is usually something in the environment that the mother may be exposed to during her pregnancy. It could be a prescribed medication, a street drug, alcohol use, or a disease that the mother has, which could increase the chance for the baby to be born with a birth defect. About 4 to 5 percent of birth defects are caused by exposure to a teratogen.
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Raymond Lewandowski, MD
Medical Director, GeneticistOfelia Rivera
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Senior Genetic CounselorCPSST - Driscoll Genetics Services
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