Maternal Serum AFP Screening FAQ's

Here are answers to some of the most common questions asked by women who are considering having serum screening for Down's syndrome and neural tube defects during their pregnancy. You are strongly encouraged to discuss the process with your physician.

 

What is the purpose of screening?

The screening process requires a small amount of blood drawn from the mother's arm which, when positive, will help identify those pregnancies that may be at increased risk for fetal abnormalities known as neural tube defects and Down's syndrome.

What is Down's syndrome?

Formerly called mongolism, this disorder is characterized by the presence of an extra chromosome, varying degrees of mental retardation, and an increased risk for physical defects.

Women of any age can give birth to a baby with Down's syndrome, though the risk does increase with increasing maternal age. For example, about 1 in 1 000 women who are 28 years old will give birth to a baby with Down's syndrome, but a woman who is 38 years old has about a 1 in 200 risk. Until recently it was thought that a woman's age was the only indicator of Down's risk. Now we can measure substances in a pregnant woman's blood and use this information as well as her age to obtain a more accurate estimate of the risk of delivering a baby with Down's syndrome.

What is a neural tube defect?

This term includes spina bifida (open spine) and anencephaly. Spina bifida results from a failure of the spinal column to fully close. The problems associated with spina bifida will vary with the size, location and nature of the opening in the spine. Problems range from mild backaches to severe mental retardation, paralysis, bowel and bladder control problems and leg deformities. Generally speaking, serum screening detects the more severe forms of spina bifida.

Anencephaly is a failure of the skull and brain to fully develop. It is incompatible with life.

What is maternal serum screening?

Maternal serum screening requires measurement of the levels of three substances in a pregnant woman's blood. The substances are alphafetoprotein (AFP), unconjugated estriol (uE 3 ) and human chorionic gonadotropin (hCG). The screening process is carried out between 15 and 20 weeks from the first day in the last menstrual period.

The levels of the three substances are used in combination with the woman's age to estimate the risk of Down's syndrome. The level of AFP alone is used to determine if there is an increased risk of a neural tube defect.

Maternal serum screening identifies women with an increased risk of Down's syndrome or neural tube defects. These women may then decide whether to have followup tests that are diagnostic. The diagnosis of Down's syndrome requires an amniocentesis (a procedure to sample the fluid around the baby). The diagnosis of neural tube defect is made by carrying out a detailed ultrasound scan as well as amniocentesis. The result of maternal serum screening is reported as either "screen negative" or "screen positive."

What is alpha-fetoprotein (AFP)?

Alpha-fetoprotein is one of the many proteins found in the blood system of a pregnant woman. Every fetus produces AFP, but when the fetus has an open spinal defect the AFP often leaks into the amniotic fluid in large quantities. Ultimately, the AFP will work its way into the mother's blood, elevating the AFP level in the mother's blood. Fetuses with Down Syndrome often produce less AFP than expected and thus the level in the mother's blood is generally lower as well.

What are my chances of having a baby with a neural tube defect?

In the Pacific Northwest, neural tube defects occur in about one in one thousand births. The cause is unknown at the present time. Over 95% of babies with neural tube defects are born to families in which there is no history of birth defects or pregnancy problems.

Will this test detect all cases of neural tube defects and Down Syndrome?

No. Studies have shown that the test will detect 70-80% of the cases of spina bifida, 90% of the cases of anencephaly, and 20-25% of the cases of Down Syndrome.

Will this test guarantee a normal baby?

No. Most birth defects are not detected by this or any other test.

What does a "screen-negative" result mean?

If the risk of Down's syndrome at the time of evaluation is found to be less than 1:1 95 and the AFP level is not high, then the screening result is called screen-negative." More than 9 out of 10 women will have a "screen negative" result.

Does a "screen-negative" result completely exclude Down's syndrome and neural tube defects?

No. The screening process will detect:

• 60% of the cases of Down's syndrome
• 90% of the cases of anencephaly
• 80% of the cases of spina bifida.

What does a "screen-positive" result mean?

A woman may be "screen-positive" for two reasons:

The risk of Down's syndrome is increased. This means that the risk of having a pregnancy with Down's syndrome at the time of testing, taking into account the woman's age as well as her blood levels of AFP, uE3 and hCG, is 1 in 195 or more.

There is an increased risk in the pregnancy of neural tube defect. The level of AFP in the mother's blood is more than two and a half times the normal value.

A screen positive result does not mean that there is an abnormality . About 5% of the women screened will have a "positive" result. Most women with a "screen positive" result will have normal healthy babies. The positive screening result only means that there is an increased risk and further tests are indicated.

What happens if the diagnostic tests are positive?

The results of the tests will be discussed with you by your physician. Information will be provided about the type of defect that has been found and how it may affect the infant. Options for further management of your pregnancy will be discussed. Usually an ultrasound will be requested to exclude twins or to more carefully estimate how far along your pregnancy is. A second blood sample may then be requested. If the second blood test is abnormal then an amniocentesis and more sophisticated ultrasound may be suggested. If you are one of these relatively few women, more extensive counseling will be provided by a genetic counselor. The diagnosis of neural tube defects usually requires a combination of AFP testing, ultrasound and amniocentesis. Amniocentesis is required to diagnose Down Syndrome.

What is amniocentesis?

Amniocentesis is a medical procedure that removes a small amount of fluid from the amniotic sac (bag of water) through a very fine needle inserted through the mother's abdomen. It is generally regarded as a safe procedure, but does have a low risk of miscarriage.

Remember

Although a positive screening result indicates a higher than average risk of Down's syndrome or neural tube defect, most women with positive screening results will have healthy, normal babies.