Screening for Birth Defects

A birth defect is a physical problem, defined as an abnormality of structure, function, or body chemistry, which is present at birth. Birth defects are relatively rare in the U.S., currently affecting only 2% to 3% of all newborn children. In most cases, the cause for the birth defect is unknown. However, some can be traced to inherited genetic factors, while others are caused by the mother’s exposure to harmful substances. There are several screening tests that are used to check for birth defects.

Some of the more common conditions found through screening are:

• Down syndrome – a chromosome problem that usually causes mental retardation and medical problems such as heart defects.
• Trisomy 18 – the presence of an extra chromosome 18. It results in severe birth defects, mental retardation, and is sometimes fatal.
• Neural tube defect – a condition where the fetal spine does not completely close. It can result in spina bifida or abnormal development of the head and brain.
• Heart defects – a condition where the chambers of the heart, or the pathways through them are not developed properly.

For All Pregnant Women

Many screening tests are routinely recommended for every pregnancy in order to obtain the best information available regarding any possible problems that the baby may have. If the screening test shows a risk for a certain disorder, further tests may be ordered for diagnosis. However, a birth defect can be present even if the screening is negative, since there are more than 4,000 possible birth defects, and not all of them can be found by testing. If a woman has certain risk factors, such as a previous child with or family history of birth defects or a genetic condition, the prenatal care provider may recommend diagnostic tests for those specific conditions.Also, parents who are members of certain ethnic groups may be advised to undergo carrier testing. A carrier is a person who does not have the disease, but carries a gene for a specific disease and therefore may pass it on to their children. If both parents are found to carry a gene for a certain genetic disorder, the family is referred to a genetic counselor to advise them on the risks to their baby.

More About Carrier Testing

Everyone carries a few abnormal "hidden" or recessive genes. These abnormal recessive genes are passed from generation to generation, usually without causing any problems. However, if both the mother and the father happen to have the same abnormal recessive gene, they could have a child with a recessive disease.When both parents carry one copy of the same recessive gene, the parents are said to be at high risk of having children with the disease. With such a high risk couple, there is a one in four chance in each pregnancy that a child will inherit two copies of the gene and be affected by the disease. Even in such a high risk couple, there is a three in four chance in each pregnancy of having a child without the disease.Carrier screening can find most couples of north European ancestry or European Jewish ancestry who are at high risk (one in four chance) for having children with cystic fibrosis. In addition, it can find most couples of east European Jewish ancestry who are at high risk of having children with Tay-Sachs or Canavan disease. Carrier screening can also find almost all couples of African, Mediterranean, Hispanic, Asian and Middle Eastern ancestry who are at high risk for having children with a serious inherited anemia.Fragile X syndrome is the second most common genetic cause of mental retardation, after Down syndrome, and occurs among people of all ethnic backgrounds. A gene which is sex-linked is responsible for causing fragile X syndrome and virtually all women who carry that gene can be found. Only women who carry an abnormal fragile X gene are at risk of having children with the disorder.

Types of Screening Tests

There are several tests that are performed during different phases of pregnancy. Your doctor will discuss with you the recommended tests for your particular circumstances. These screening tests are safe for you and your baby.

The First Trimester Screen - is a noninvasive screen that combines a maternal blood screening test with an ultrasound evaluation of the fetus to identify risk for specific chromosomal abnormalities, including

Down syndrome and Trisomy-18. In addition to screening for these conditions, the ultrasound portion of the test (known as the nuchal translucency) can assist in identifying other significant problems, such as cardiac disorders. The combined accuracy rate for the screen to detect the chromosomal abnormalities mentioned above is approximately 85% with a false positive rate of 5%.

This means that:

Approximately 85 out of every 100 babies affected by the abnormalities addressed by the screen will be identified.
Approximately 5% of all normal pregnancies will receive a positive result or an abnormal level.

Second Trimester Screening - Between 15 to 20 weeks of pregnancy, a test called "multiple marker screening" is offered to screen for Down syndrome, trisomy 18, and neural tube defects. This test measures the level of three or four specific substances in your blood. The triple screen measures the levels of three substances in your blood: alpha-fetoprotein (AFP), human chorionic gonadotropin (hCG), and unconjugated estriol (uE3). Some tests measure the hormone inhibin A as well — this is called a quadruple screen.

Based on the results of the levels of these substances, the doctor can assess your risk for certain problems. For example, a high AFP level can mean several things. A baby produces AFP throughout gestation, and a certain amount of it should cross the placenta into the mother's bloodstream at each stage.

If there's more AFP than expected, it may mean that you're carrying more than one baby or that your baby is older than your practitioner thought. But in some cases, it's a sign of an abnormal opening in the baby's spine (spina bifida), head, or abdominal wall that's allowing AFP to leak out. In rare cases, it can also signify a problem with the baby's kidneys. And in some cases, it doesn't mean any of those things.

Low AFP, low estriol, high hCG, and high inhibin A are associated with a higher risk for Down syndrome. Low levels of the first three mean your baby has an increased risk of having trisomy 18, a more severe and less common chromosomal abnormality.

Keep in Mind It is very important to remember what a screening test is before getting one performed. Screening tests compare a number of different factors, including age, ethnicity, and results from blood tests, and give an estimate of a baby’s risk of being born with a problem. These tests alone can’t diagnose a problem; they are only indicators of if and when certain additional testing should be done.