Down syndrome, also called "trisomy 21," occurs when there is an extra chromosome 21 present in cells of the fetus. All of our genes are packaged into chromosomes which control all of our genetic characteristics. Chromosomes are present in pairs.
Normally, a baby is born with 46 chromosomes in every cell, including two copies of the number 21 chromosome. A child with an extra chromosome 21 will have mental retardation and the characteristic appearance of Down syndrome. About 40 percent of children with Down syndrome are also born with serious heart problems.
- Why should I consider being screened for Down syndrome?
Down syndrome occurs in approximately 1 in 800 births. The older a woman is at the time of her delivery, the greater the chance that she will have a baby with Down syndrome. However, a woman at any age can have a baby with Down syndrome. By combining the results of the Multiple Marker Screening (MMS) blood test, with your age and other factors, you will receive a more accurate assessment of your individual risk for having a baby with Down syndrome.
- Should I have this test if there has never been anyone in our family with own syndrome?
In over 95 percent of the families in which a child is born with Down syndrome, there has never been anyone else in the family with this problem. A very small percentage of Down syndrome is inherited in families. If you or your partner have a history of Down syndrome in the family, please let your doctor or genetic counselor know prior to having this test.
- How accurately does the MMS test screen for Down syndrome?
The MMS test detects approximately 70 percent of Down syndrome pregnancies. If your test result shows a risk for Down syndrome that is comparable to the risk of a 35-year-old woman (a one in 270 chance), you will be counseled by your doctor and our genetic counselors about the result. If your test result shows a risk lower than that of a 35-year-old woman, the report will be sent to your doctor. If you wish to know the numerical risk estimated by the test, please ask your doctor.
- What if the screening test shows that my risk for Down syndrome is equal to or greater than that of a 35-year-old woman?
The first step is to have an ultrasound examination. Sometimes the blood test results are higher or lower than we expect simply because your pregnancy is not quite as far along or is more advanced than we calculated by your last menstrual period. In this case, the expected date of your delivery and the interpretation of your test may be changed after the ultrasound scan. If your risk for Down syndrome is still increased, you may decide to have an amniocentesis to test for Down syndrome in the fetus. Amniocentesis is done by removing a small amount of the amniotic fluid surrounding the fetus and checking the chromosomes in the cells that are in this fluid for the extra chromosome 21.
New Down syndrome screening tests continue to become available and can be performed in the first trimester. These tests include a combination of enhanced ultrasound and blood tests. Talk to your doctor about the latest technology available to detect Down syndrome.
- If I am over 35 years of age, should I consider having the MMS blood test first to identify my risk of having a child with Down syndrome before deciding about an amniocentesis?
The current recommendation for prenatal care of women over 35 years of age is that they should be offered amniocentesis. MMS is not a substitute for amniocentesis. However, some patients find that the MMS blood test helps them make a decision regarding whether an amniocentesis is the right choice for them. These options require more extensive counseling by our genetic counselors before you make this important decision. If you will be over 35 years of age when you deliver, please contact one of our genetic counselors for more information.
- What if amniocentesis shows that my baby will have Down syndrome or some other serious birth defect?
If your baby is found to have a serious birth efect, you will receive counseling about what this will mean for you and your family.
- If my blood test is normal, does that mean that my baby will be normal?
The MMS test identifies seven out of 10 cases f Down syndrome (70 percent). Because MMS is a screening test, some cases of Down syndrome will be missed. The AFP part of the MMS test also screens for neural tube defects, such as spina bifida. There are other types of birth defects that are not detected by this test. Therefore, a normal test does not guarantee a normal baby.
Penn Ob/Gyn Care physicians and counselors provide prenatal diagnosis, consultation and care at various locations throughout the Philadelphia region. Please call 800-789-PENN (7366) for more information.