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• 1st Trimester Genetic Risk Assessment
• Maternal Screening
• Chorionic Villous Sampling
• Genetic Amniocentesis
• Cystic Fibrosis Carrier Screening
• Integrated Prenatal Screening

Maternal Screening

All pregnant women are now being offered maternal serum screening (also known as AFP testing, triple or quad screening) for certain birth defects.  It is important that you understand this screening process so you can decide whether or not to be tested.  We encourage you to talk with your health care provider about this process.

What is the purpose of this screening?
Your unborn baby makes some proteins which can be detected in your blood during pregnancy.  Measuring certain proteins at 15 to 20 weeks from the first day of the last menstral period can help identify whether you may be at increased risk to have a baby with a neural tube defect (spina bifida), Down syndrome, or trisomy 18.

What is neural tube defect?
A neural tube defect occurs very early in pregnancy when the cells that become the skull, brain, and spinal column fail to form a closed tube.  Neural tube defects occur in about one in 1,000 births.  Often couples who have a baby with a neural tube defect have no family history of these problems.

If this tube does not form correctly at the top, the brain and skull will not develop.  This is called anencephaly.  Babies with anencephaly do not survive.

If there is an opening along the lower portion of the tube, the spine does not close completely.  This is called spina bifida.  Size and location of the opening determine how severe the spina bifida is.  Complications may include problems with walking and bowel and bladder control.

What is Down Syndrome?
In each of our cells, we have 46 chromosomes (23 pairs).  They give instructions for our development.  Down syndrome results when there is an extra number 21 chromosome (three, instead of a pair).

Any woman at any age can have a baby with Down syndrome, although the risk increases with the age of the mother.  Babies with Down syndrome usually have a mild to moderate degree of mental retardation, as well as other physical problems.

What is trisomy 18?
Trisomy 18 results when there is an extra number 18 chromosome.  Babies with trisomy 18 have more severe birth defects and mental retardation.

How does maternal serum screening work?
This screening measures the levels of up to three different proteins from the unborn baby or placenta that circulate in your blood:

• Alpha-fetoprotein (AFP)
• Unconjugated estriol (uE3)
• Human chorionic gonadotropin (hCG)
• Inhibin

Elevated levels of AFP may mean that the unborn baby has a neural tube defect.

Low levels of all three proteins may mean the unborn baby has trisomy 18.

In pregnancies where the unborn baby is affected with Down syndrome, there may be a special pattern in the levels of all three proteins:

• AFP and uE3 are low
• hCG and inhibin are elevated

What does it mean if my screen is negative?
About 95 out of 100 women screened will have a negative result.  If the Down syndrome risk is found to be less than 1 in 365 (equal to a 35-year-old woman's risk) and the AFP level is not elevated, then the result is reported as screen negative.

Unfortunately, a negative screen does not guarantee the baby will be normal, because maternal serum screening does not identify all unborn babies with these problems.  It leads to the detection of:

• 6 out of 10 unborn babies with Down syndrome
• 8 out of 10 unborn babies with spina bifida
• 9 out of 10 unborn babies with anencephaly
• 8 out of 10 unborn babies with trisomy 18

What does it mean if my screen is positive?
In 5 out of 10 of every 100 pregnant women tested, the result is "screen positive."  This result does not necessarily mean the baby has a problem—it means that the unborn baby is at increased risk for a problem.

• At the University of Washington Medical Center, a screen positive means:
• The risk of that the unborn baby has Down syndrome is greater than or equal to 1 in 365 (a 35-year-old woman's risk to have a baby with Down syndrome), OR
• The AFP is more than 2.5 times the normal value, OR
• The risk that the unborn baby has trisomy 18 is increased because all three proteins are low (approximately half the normal values)  OR
• Simply that the woman may be 35 or older

Whether or not you have more testing is up to you.  You will need to decide if the risk seems high enough to consider diagnostic tests such as ultrasound or amniocentesis.

Remember, The maternal serum screening test is not a diagnostic test.  It will not tell you whether or not your unborn baby has or doesn't have a problem. It will simply indicate the chance that the baby has a problem.

Keep in mind that the vast majority of women with "screen positive" results do not have babies with Down syndrome, trisomy 18, or a neural tube defect.

Who might decide NOT to have screening?
Knowing about these problems ahead of time may change the management of your pregnancy and delivery.  However, some couples have decided that they would not consider having an amniocentesis for any reason and/or just do not want to know during the pregnancy if their baby has one of these problems. If that is the case, you should not have your blood drawn.

How confident can I be that the results are accurate?
The University of Washington Medical Center employs experienced registered medical technologists to perform these tests.

Our laboratory participates in voluntary quarterly proficiency surveys sponsored by the College of American Pathologists and has a strict quality control program to assure reliable results.

Lab personnel meet frequently with health care providers and genetic counselors to ensure clear communication about laboratory testing and clinical interpretation of test results.

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Last Updated:
4/12/07