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What is a serum screening blood test during pregnancy?

The blood tests that are offered during pregnancy are performed for many reasons. Some to see whether you are low in iron, have diabetes and other reasons but the serum screening blood test, done around 15 to 18 weeks is specifically performed to screen for Mom’s that may be at risk of carrying a baby with a problem.

Prenatal serum screening blood tests can measure between two, three or four substances within the Mother’s blood to predict whether your baby is at risk for Down’s syndrome or a neural tube defect such as spina bifida. These tests are commonly carried out between 15 to 18 weeks.

It is very important to remember what a screening test is before have one performed which will also help alleviate some of the anxiety that can accompany test results. These tests, called screening tests are just that…a screening, and not a definite diagnosis. These tests try to identify those babies that may be at a higher risk for an abnormality but this does NOT diagnose a problem. The test only gives us an indication or risk level and determine a need for further investigation or not.  It is important to discuss the risks and benefits of testing at length with your doctor before pursuing any screening. You may not want to pursue any further investigations and this is important to establish. Whatever decision you feel most comfortable with is the right one, but it is always good to speak with your doctor.

There Are Three Different Serum Screen Blood Tests:

Some facilities may only offer one of the following tests.

1. The double test measures the level of two substances in the Mother’s blood, alpha-fetoprotein and free beta-HCG.

2. The triple test measures the level of three substances in the Mother’s blood, alpha-fetoprotein, free beta-HCG, and oestriol.

3. The quadruple test measures the level of four substances in the Mother’s blood, alpha-fetoprotein, free beta-HCG, oestriol and inhibin A. In my opinion, the quad screen in place of the double or triple test increases the likelihood of identifying pregnancies at risk for Down Syndrome and also lowers the false positive rate.

  • Alpha-fetoprotein is a protein that is produced by the fetus
  • Free beta-HCG (beta-human chorionic gonadotropin) is a hormone produced within the placenta
  • Oestriol is an estrogen produced by both the fetus and the placenta
  • Inhibin A is a protein produced by the placenta and the ovaries

Not every women or couple choose to have the serum blood screening done. But, the quad serum screen may be recommended for women who:

  • Have used certain harmful medications or drugs during early pregnancy
  • Are 35 years or older
  • Have a family history of birth defects
  • Have had a viral sickness during pregnancy such as toxoplasmosis, rubella, cytomegalovirus
  • Are diabetic and are on insulin

These substances (Alpha-fetoprotein, Free beta-HCG, Oestriol, Inhibin A) that are carried in the Mother’s blood give an indication of a possible risk that Mother is carrying a fetus with an abnormality but one must remember that if one of these tests is positive it does not mean that your baby has an abnormality just that there is a higher risk of one.

What does the quad screen test check ?

The quad blood screen measures high or low levels of alpha-fetoprotein , abnormal levels of hCG and oestriol, and high levels of Inhibin-A. These results are combined with the mother’s age and ethnicity in order to assess probabilities of any potential genetic disorders. These results are to be interpreted by your doctor.

High levels of alpha-fetoprotein may suggest that the developing baby has a neural tube defect such as spina bifida or anencephaly (an-en-sef-uh-lee), which is a baby that is developing without the top of the head.

Low levels of alpha-fetoprotein  and abnormal levels of hCG and estriol may indicate that the developing baby has Trisomy 21(Down syndrome), Trisomy 18 (Edwards Syndrome) or another type of chromosome abnormality.

I want to emphasize that the most common reason for elevated alpha-fetoprotein levels is due to inaccurate dating of the pregnancy. This means that while you may have thought you were 15 weeks pregnant you may actually be 17 weeks pregnant, which explains the adjusted level of alpha-fetoprotein levels in your blood.

How Are The Tests Performed?

These tests are carried out by taking a sample of Mother’s blood, which only takes a few minutes. The blood is then sent to a special laboratory to be analyzed. It may take several days to receive the results.

The blood results are not the only element of this screening process that determine your risk factor. Ethnic background, age of the Mother,  gestational age of the baby and the levels of these substances togeather are entered into a computer algorithm to calculate your individual risk.

Except for the discomfort of drawing blood, there are no known risks or side effects associated with the quad screen test.

What do results mean?

It is important to remember that the quad screen is a screening test and not a diagnostic test. This test only notes that a mother is at risk of carrying a baby with a genetic disorder. Many women who experience an abnormal test result go on to deliver healthy babies.

Abnormal test results warrant additional testing in order to make a diagnosis. A more conservative approach involves performing a second quad screen followed by a high definition ultrasound. If the testing still maintains abnormal results, a more invasive procedure such as amniocentesis may be performed.

Any invasive procedure should be discussed thoroughly with your partner, your doctor and you may require additional counselling and discussions with a counsellor or social worker.

What are the reasons for further testing?

Reasons to pursue further testing vary from person to person and couple to couple. Further testing can confirm a diagnosis and then offer information and opportunities to:

  • Research opportunities for corrective surgeries for things such as spina bifida
  • To contact support groups to help you plan for a child with special needs
  • To prepare for lifestyle change that might include home renovations
  • Deciding whether to continue the pregnancy

Some individuals or couples may decide not to do any further testing for personal, religious or moral reasons and still other parents opt for no further invasive testing because of the potential risk of losing the pregnancy. It is important to discuss the risks and benefits of testing thoroughly with doctor as these decisions are important and your doctor can help you evaluate what tests are right for you and the benefits versus the risks.


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