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Most tests are done to see if you have a particular disease or condition. These are called diagnostic tests. A screening test is different. Screening tests show if a person’s risk is high or low for a specific disease or condition. If the risk is high, a person can then choose to have diagnostic testing.
Screening tests are common for pregnant women. The most common of these tests estimate the risk of the fetus having either Down syndrome or spina bifida (SPY-nuh BIF-ih-duh).
Neural tube defects include defects of the brain and spine. One defect prevents the skull bones from closing. This is called anencephaly (an-en-SEF-uh-lee). Babies with anencephaly are usually stillborn or die shortly after birth.
The other serious open neural tube defect is caused by an open spine. This is called spina bifida. Some babies with spina bifida have paralysis. They also may have other physical or intellectual disabilities. One to two babies out of every 1,000 live births have an open neural tube defect.
Down syndrome is caused by an extra number 21 chromosome. Normally, people have two copies of each chromosome. People with Down syndrome have three copies of the number 21 chromosome. This syndrome is sometimes called trisomy 21. It results in mental and growth retardation. It may cause heart defects and other physical problems. Down syndrome occurs in about 1 out of every 700 live pregnancies. The chance of a child having Down syndrome is greater in older mothers.
For example, a 25-year-old woman has a 1 in 1,250 chance of having a baby with Down syndrome. A 35-year-old woman has a 1 in 380 chance, and a 45-year-old woman has a 1 in 30 chance.
Many factors are considered when your screening is analyzed, including:
Multiple marker screening can identify about 70 percent of pregnancies with Down syndrome and about 80 percent of pregnancies with spina bifida. But a negative screening result does not prove that the baby will not have either of these conditions. Even with a negative result, there is still a chance that the baby will have Down syndrome, spina bifida, or other genetic disorders.
Most women who are identified as “increased risk” by multiple marker screening will have a normal, healthy baby. An increased risk result does not mean that your baby has Down syndrome, or any other problem. It only means that further testing may be appropriate. Other tests may include a second multiple marker screening, an ultrasound, or amniocentesis.
Genetic counselors are available to discuss with the parents the decision to have further testing. This decision should be based on a number of factors, including the risk of birth defects and the risks of the procedure.