INTRODUCTION — This topic provides information about prenatal screening for Down syndrome to help you decide if you want to undergo this test.
If you are certain that you want to have screening, more detailed information is available separately. (See "Patient information: First trimester and integrated screening for Down syndrome" and see "Patient information: Second trimester screening for Down syndrome").
WHAT IS DOWN SYNDROME? — The first decision you need to make is whether you want to know if your developing baby has Down syndrome before birth. It may help to review some facts about the condition.
- Down syndrome is caused by an extra number 21 chromosome.
- It occurs in about 1 in 500 pregnancies.
- People with Down syndrome have mild to moderate mental retardation, meaning that the person can often do things independently; however, most need supervision throughout their lives.
- People with Down syndrome have characteristic facial features (show picture 1), meaning that their facial features are similar to those of other people with Down syndrome.
- People with Down syndrome may have birth defects, such as problems with how the heart or intestines develop. Other medical problems can also develop.
- The average lifespan for an individual with Down syndrome is about 50 to 60 years.
Could my baby have Down syndrome?
- A woman of any age can have a child with Down syndrome, but the risk increases as the mother's age increases.
- Down syndrome usually does not run in families, except in rare cases. You should inform your health care provider if there is a family history of Down syndrome.
WHAT INFORMATION DOES A SCREENING TEST PROVIDE? — A screening test will tell you the chances of having a certain condition, such as a baby with Down syndrome. Screening tests for Down syndrome do not determine whether the baby actually has Down syndrome; rather, they identify women who are at increased risk of having an affected child.
Most Down syndrome screening tests require a sample of blood from your arm and some also involve an ultrasound exam to look at your developing baby. These tests pose no risk to the pregnancy.
Your result will either be "screen positive", meaning that your chance of having a baby with Down syndrome is HIGHER than the test cut-off level, or "screen negative", meaning that your chance of having a baby with Down syndrome is LOWER than the test cut-off level. The test cut off level is determined by the laboratory and is stated as a risk or likelihood.
If your result is screen positive, diagnostic testing (chorionic villus sampling or amniocentesis) is the next step. These diagnostic tests are associated with a small risk of miscarriage, but will tell you for sure if your developing baby actually has or does not have Down syndrome. Most women with a screen positive result have a healthy baby.
If your result is screen negative, a diagnostic test is usually not performed since the risk of Down syndrome is low. However, the risk is not zero; occasionally, a woman with a screen negative result will have a baby who has Down syndrome.
The decision to have a prenatal screening test for Down syndrome is yours and depends upon your wishes, values, and beliefs. There is no right or wrong choice; you decide what is best for you and your family.
WHO IS OFFERED A SCREENING TEST FOR DOWN SYNDROME? — The American College of Obstetricians and Gynecologists recommends that all pregnant women, regardless of age, be offered the opportunity to have a screening test for Down syndrome before 20 weeks of pregnancy. Screening tests for Down syndrome are voluntary, meaning that it is your choice whether to have or not have these tests.
In addition, all women should have the option of having diagnostic testing rather than screening tests. Diagnostic testing are also offered to women of any age who have a higher risk of having baby with a chromosome problem, such as Down syndrome, for any one of the following reasons:
- The woman has previously had a baby with a chromosomal problem, such as Down syndrome.
- The developing baby is found to have a structural problem by ultrasound (such as a heart defect).
- The developing baby has ultrasound findings that increase the risk for a fetal chromosome problem such as Down syndrome (for example, increased nuchal translucency or a cystic hygroma).
- One of the parents is known to have a rearrangement or change in one of their own chromosomes.
DECIDING TO HAVE A SCREENING TEST
Why should I have a screening test? — These are some of the reasons that women and their partners choose to have screening for Down syndrome.
- I want as much information as possible during pregnancy about the health of my developing baby.
- If my baby has Down syndrome, I want to know while I am pregnant so I can learn as much as possible about the condition before the baby is born.
- I am planning to deliver my baby in a community hospital, so if my baby has serious birth defects associated with Down syndrome (eg, heart or intestinal abnormalities), I would rather deliver at a hospital with a special care nursery.
- I have been anxious since I learned I was pregnant and if I have a screen negative result, I believe it will help ease my anxiety.
- I want to consider all of my options. If my developing baby has Down syndrome I would want the option to terminate the pregnancy.
- I am not sure what I would do, or how I would feel, if my developing baby has Down syndrome. I am going to take it one step at a time. If my screening test comes back saying I am screen positive, I will decide at that time if I want to have a diagnostic test.
Why might I choose not to have a screening test? — These are some of the reasons that a woman and her partner might choose NOT to have screening.
- I have decided that 'whatever will be, will be' and I will wait until the baby's birth to find out if the baby is healthy.
- I do not want to be faced with decisions about my unborn baby. Because of religious or personal beliefs, I would never terminate an affected pregnancy.
- Since I know I would never have a diagnostic test with even a small risk of a miscarriage, I do not want to have a screening test.
Some common myths about screening for Down syndrome — Some of the reasons women decide whether or not to have screening are based on incorrect information, such as:
- Myth — My baby won't have Down syndrome because I am young, I exercise, and I am healthy.
- Fact — A woman of any age can have a baby with Down syndrome, regardless of her health.
- Myth — My baby won't have Down syndrome because I do not drink or smoke.
- Fact — Avoiding alcohol or tobacco during pregnancy is very important for the health of you and your baby; however it does not affect the chance that your baby will have Down syndrome.
- Myth — My baby won't have Down syndrome because no one in my family or the father of the baby's family has Down syndrome.
- Fact — Down syndrome usually does not run in families. If you have a family history of Down syndrome, you should talk to your health care provider or a genetic counselor to discuss if it will increase your risk of having a baby with Down syndrome.
- Myth — I should not have screening for Down syndrome unless I know that I would terminate the pregnancy if Down syndrome is detected.
- Fact — Many individuals who would not terminate their pregnancy choose to have screening. These individuals want information about their unborn baby's health before birth.
- Myth — My friend told me that if I have a screening test, it will come back 'positive' since most people who have the test end up with a 'positive' result.
- Fact — Most people who have a screening test will have a 'negative' result. The percentage of people who will come back 'positive' depends on what screening test is performed. At least 93 percent of women who take the test will have a screen negative result.
WHAT SCREENING TESTS ARE AVAILABLE? — There are several different screening tests available. Some important considerations include the following:
- How far along in pregnancy are you?
- What screening tests are available in your area?
- What, if any, diagnostic tests (CVS or amniocentesis) are available in your area?
First trimester screening — Screening for Down syndrome can be done as a first trimester-only test. This involves taking a blood sample from you and having a specific ultrasound measurement of the developing baby, called nuchal translucency. This type of screening test is done between 11 and 13 completed weeks of pregnancy. (See "Patient information: First trimester and integrated screening for Down syndrome").
Integrated screening — Screening for Down syndrome can be done by collecting information about the pregnancy during the first and second trimesters and then 'integrating' all of this information into a single risk for Down syndrome.
The full integrated test is done by collecting a blood sample between 10 and 13 completed weeks of pregnancy and then having an ultrasound measurement of the back of the neck of the developing baby; this is called a nuchal translucency measurement. Another blood sample is collected between 15 and 22 completed weeks. The laboratory uses all of this information to estimate the risk for Down syndrome. You will not get any results until both blood tests have been completed.
The serum-only integrated test (using blood tests only, no ultrasound examination) can be done if nuchal translucency ultrasound is not available in your area. You will not get any results until blood tests from the first and second trimester have been completed.
There are some variations of the full integrated test that may be available in some areas:
- Contingent screening — This is a full integrated test in which women with the highest and lowest first trimester risks are informed of their result. Women with the highest risks may choose early diagnostic testing and women with the lowest risks typically have no further testing. Those with a first trimester risk between high and low are advised to complete the integrated test by having a blood sample drawn in the second trimester. The quality of the contingent test has not yet been proven in clinical trial and is not yet recommended for routine use.
Second trimester screening — Screening for Down syndrome can be done by performing a blood test in the second trimester of pregnancy, between 15 and 22 completed weeks. This is the same test that is performed as the second part of integrated testing, and includes a screening test for neural tube defects (spina bifida). (See "Patient information: Second trimester screening for Down syndrome").
Which screening test should I choose? — The "best" screening test depends upon your values and preferences. You can use the following statements to help guide your decision (show table 1).
WHERE TO GET MORE INFORMATION — Your healthcare provider is the best source of information for questions and concerns related to your medical problem. Because no two patients are exactly alike and recommendations can vary from one person to another, it is important to seek guidance from a provider who is familiar with your individual situation.
This discussion will be updated as needed every four months on our web site (www.uptodate.com/patients). Additional topics as well as selected discussions written for healthcare professionals are also available for those who would like more detailed information.