Maternal serum screening

Maternal serum (blood) screening can identify pregnancies that are at a higher than average risk for certain birth defects.

Maternal serum alpha-fetoprotein (MSAFP) screening was the first test to be used in this manner. MSAFP measures the amount of a certain protein made by the baby's liver. This protein passes into the amniotic fluid (the fluid that surrounds and protects the baby in the womb). A small amount is absorbed into the mother's bloodstream.

Today, two additional tests are often done with MSAFP to increase the test's accuracy. This combination test is referred to as the maternal serum triple test, triple screen or MSAFP+ test. It examines maternal serum alpha-fetoprotein, human chorionic gonadotropin (a hormone produced by the placenta) and estriol (a hormone produced in the placenta and the baby's liver).


Purpose
Maternal serum screening can't diagnose birth defects. Instead, it compares your risk of having a baby with certain defects to that of other women your age. It's useful in screening for:
Neural tube defects, which affect the tissue that becomes the brain and the spinal cord. An example of this type of defect is spina bifida, in which a part of the baby's spine remains open, leaving the spinal cord unprotected. If the defect is small there may be no symptoms, but severe defects may cause paralysis of the legs. Another neural tube defect is anencephaly, the absence of part or all of the brain. This condition is fatal before or soon after birth.

Down syndrome (trisomy 21), a combination of defects that lead to some degree of mental retardation and characteristic facial features. The heart, eyes and hearing may also be affected.

Trisomy 18, a combination of defects that can affect many organs and cause abnormal development and severe mental retardation. It's often fatal before age 1.

Other birth defects, including defects of the abdominal wall, kidney and bowel.
The results of maternal serum screening tests can help you and your doctor decide if you would benefit from additional screening tests to rule out or identify specific birth defects.


Indications
The triple screen is typically offered to all pregnant women. In fact, offering the test is legally required in some states. You have the choice to take the test or not, however. Your decision may depend on your risk factors as well as your personal wishes. Some mothers want as much information as possible to help prepare themselves for the birth of a baby with a defect. For others, the results can help them decide whether to continue the pregnancy.
If you have an increased risk of having a baby with a birth defect, you may instead request to have other tests that can more accurately diagnose a defect, such as:

amniocentesis (withdrawing a sample of amniotic fluid -- the fluid that surrounds and protects the baby in the womb -- for examination)
chorionic villus sampling (taking a small sample of the chorionic villi -- small projections of placental tissue that attach the membrane of the embryo to the wall of the uterus -- for microscopic evaluation)
ultrasound (using sound waves to create an image of the baby)
You may be at increased risk of having a baby with a birth defect if:

You're age 35 or older.
You have a biological child with a genetic disorder or neural tube defect, or such a defect was detected in a previous pregnancy.
You or your partner have a genetic disorder or could be carriers of one.
You have a family history of a genetic disorder.
You've had several miscarriages.

Patient preparation
There's no special preparation for this blood test.

Procedure
Maternal serum screening is typically done between the 15th and 20th week of pregnancy. The procedure is quick and simple -- the doctor simply takes a sample of your blood.
The amount of each substance included in the screen is measured. This information is interpreted in relationship to your age, weight and race, in addition to the baby's age. If you have the triple screen, the levels of the three substances are then compared to each other. Test results are usually available within a week.


Postprocedure care
As many as 100 out of every 1,000 women have abnormal maternal serum screening results. Only 10 of those 100 have babies with birth defects, however. Sometimes test results are abnormal if the baby is older or younger than originally thought or more than one baby is present. It's important to see your doctor for an interpretation.
If you have an abnormal reading, more testing may be needed. For example, the doctor may suggest an ultrasound to determine the baby's age, the presence of twins, or whether there are any identifiable malformations. The screening results will be recalculated if the baby's original age is found to be inaccurate or twins are present. If the baby is found to be younger than 15 weeks, the doctor may recommend a repeat maternal serum test after 15 weeks.

If the ultrasound doesn't provide answers, your doctor may suggest an amniocentesis. Measuring the alpha-fetoprotein as well as other substances found in the amniotic fluid can help the doctor determine if a neural tube defect actually exists. Other tests of the amniotic fluid can signal other types of birth defects. If the doctor finds no clear explanation for the abnormal screening result, your pregnancy will be monitored closely.


Benefits
Maternal serum screening results may be one step in the early diagnosis of a birth defect. This information can be used to determine treatments that, in some cases, can be done before birth. This information can also help you prepare for the birth of a baby with a birth defect, choose an appropriate hospital for the baby, or decide whether to continue the pregnancy.

Considerations
It's important to remember that maternal blood screening for an increased risk of birth defects is voluntary. Make sure you understand the benefits and limitations of screening.
You may have a false-positive result, which means the test result is abnormal but the baby doesn't have a birth defect.

You could also have a false-negative result, which means a birth defect or abnormality goes undetected. If you decide to skip any further testing, the uncertainty may be difficult.

Discuss any concerns with your doctor before making a decision about testing. If you need additional testing, make sure you understand the risks and benefits.