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.
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. Most babies are born healthy. In fact, 97 out of 100 babies are born healthy. Anytime a couple becomes pregnant, there is a 3 to 4 percent chance that their baby will have a birth defect. 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 in family members or the parents themselves, 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 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.
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 7 percent of birth defects are caused by exposure to a teratogen.
|Causes of Birth Defects||Frequencies|
|Single Gene Defects||7.5 percent|
|Chromosome Abnormalities||6.0 percent|
|Multifactorial Inheritance||20.0 percent|
Why are birth defects a concern?
Although some birth defects have a single abnormality, others have abnormalities in multiple body systems or organs. Birth defects may cause life-long disability and illness, and with some, survival is not possible.
Some birth defects, such as mental retardation, are non-treatable disabilities. However, many physical defects can be treated with surgery. Repair is possible with many defects including cleft lip or palate, and certain heart defects.
How are birth defects diagnosed?
Many birth defects can be diagnosed before birth with special tests (prenatal diagnosis). Chromosomal abnormalities such as Down syndrome can be diagnosed before birth by analyzing cells in the amniotic fluid or from the placenta. Fetal ultrasound during pregnancy can also give information about the possibility of certain birth defects, but ultrasound is not 100 percent accurate, since some babies with birth defects may look the same on ultrasound as those without problems. A chromosome analysis, whether performed on a blood sample or cells from the amniotic fluid or placenta, is over 99.9 percent accurate.
Tests that help screen for birth defects include the following:
- alpha-fetoprotein - this blood test measures the levels of alpha-fetoprotein (AFP), a protein released by the fetal liver and found in the mother's blood. AFP is sometimes called MSAFP (maternal serum AFP). AFP screening may be included as one part of a two, three, or four-part screening, often called a multiple marker screen. The other parts may include the following:
- hCG - human chorionic gonadotropin (hCG) is a hormone secreted by the early placental cells. High hCG levels may indicate a fetus with Down syndrome (a chromosomal abnormality that includes mental retardation and distinct physical features).
- estriol - a hormone produced by the placenta and by the fetal liver and adrenal glands. Low levels may indicate a fetus with Down syndrome.
- inhibin - a hormone produced by the placenta.
- chorionic villus sampling (CVS) - a prenatal test that involves taking a sample of some of the placental tissue. This tissue contains the same genetic material as the fetus and can be tested for chromosomal abnormalities and some other genetic problems. Testing is available for other genetic defects and disorders depending on the family history and availability of laboratory testing at the time of the procedure. In comparison to amniocentesis (another type of prenatal test), CVS does not provide information on neural tube defects such as spina bifida. For this reason, women who undergo CVS also need a follow-up blood test between 16 to 18 weeks of their pregnancy, to screen for neural tube defects.
- amniocentesis - a procedure used to obtain a small sample of the amniotic fluid that surrounds the fetus to diagnose chromosomal disorders and open neural tube defects (ONTDs) such as spina bifida. Testing is available for other genetic defects and disorders depending on the family history and availability of laboratory testing at the time of the procedure. An amniocentesis is generally offered to women around 14 weeks of pregnancy for those women who are at increased risk for chromosome abnormalities, such as women who are over age 35 years of age at delivery, or those who have had an abnormal maternal serum screening test, indicating an increased risk for a chromosomal abnormality or neural tube defect.
- ultrasound - a diagnostic technique that uses high-frequency sound waves to create an image of the internal organs. Many birth defects can be detected with ultrasound.
Sometimes, birth defects are not diagnosed until physical examination of the baby after birth. To confirm the physical findings, a small blood sample can be taken and the chromosomes can be analyzed. This information is important in determining the risk for that birth defect in future pregnancies.
Prevention of birth defects:
Research is ongoing to find and treat the causes of many birth defects. Immunizations of the mother against certain infections, such as rubella, can prevent infection. Much has been learned about the dangerous effects of alcohol on the developing baby and women are now advised to not drink during pregnancy. In recent years, a strong link has been discovered between the lack of the B-vitamin folic acid and the development of neural tube defects such as spina bifida. Taking a vitamin containing sufficient folic acid before conception and in early pregnancy can often help prevent many serious defects.
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