Maternal Serum Screening

Maternal Serum Screening Test For Down Syndrome

Maternal Serum Screening Test is a voluntary blood test conducted between 15 and 20 weeks of pregnancy to aid in the risk assessment of Down Syndrome, a congenital disease caused by a defect in the chromosomes.

What Is The Test For?

The Maternal Serum Screening test is used to identify pregnancies in which the risk of Down Syndrome is significant enough to justify considering an amniocentesis.

How Does It Work?

Conducted between 15 to 20 weeks of pregnancy, the test measures the amount of AFP (alpha-fetoprotein) and hCG (human chorionic gonadotrophin). These substances are produced by the foetus and the placenta, and can be detected in the mother's blood.

Together with mother's age, the measurements of AFP and hCG are used to calculate her risk of carrying a baby with Down Syndrome. The test provides the mother with her individual risk of having a baby with Down Syndrome. This numerical risk is classified into either "low-risk" or "high-risk" using a cut-off risk of 1 in 250 to help mothers decide whether or not to go for an amniocentesis.

Why Measure These Substances?

We know that women with low AFP in their blood may have a higher than average risk of having a baby with Down Syndrome. It appears that babies with Dow Syndrome tend to produce less AFP than normal babies, and therefore less of it is transferred to the mother.

The level of hCG tends to be higher when Down Syndrome is present.

The reason of these differences is not known. However, a delay in maturity of Down Syndrome babies may be a cause as these two substances are associated with foetal maturity.

The measurement of these substances helps doctors to estimate more accurately a mother's risk of carrying a baby with Down Syndrome.

What Does The Test Result Mean?

In pregnancies with increased risk for Down Syndrome, the amount of AFP is lower and the amount of hCG is higher.

A "high risk" result does not mean that the baby has Down Syndrome. It only means that there is an increased risk that causes enough concern for your specialist to offer an amniocentesis (a diagnostic test for Down Syndrome) to confirm or refute the presence of a Down Syndrome pregnancy.

Likewise, a "low risk" result is not a guarantee that your baby will be healthy. If your test shows a "low risk" result, it means there is a lower chance of a Down Syndrome baby being born, so no further action is usually required.


Your individual risk is quoted as 1 in 250. What this means is that out of 250 pregnancies, only one of them will be affected and 249 will be normal.

If your risk is shown to be more than 1 in 250 of having a baby affected by Down Syndrome (for example 1 in 50), you will be offered an amniocentesis to confirm or refute the presence of Down Syndrome.

Why Is The Serum Test Done Between 15 And 20 Weeks of A Pregnancy?

Experience has shown that the blood test works best between 15 and 20 weeks of pregnancy. Having the test conducted during this period allows time for intervention, if needed, before the pregnancy is more advanced. It has also been shown that the risk calculated is more accurate if an ultrasound scan has been done to confirm the age of the pregnancy and to exclude multiple pregnancies.

Can The Test Screen For Other Types Of Abnormality?

Yes. The test can also indicate if there is an increased risk for other abnormalities, particularly open spina bifida, a defect of the backbone. This is indicated by the presence of a high AFP level. When this happens, a detailed ultrasound scan will usually be arranged for further investigation.

What Happens When An Abnormality Is Discovered After Amniocentesis Or During An Ultrasound Scan?

Your specialist will provide you with detailed information about the abnormality discovered and how it may affect your baby. You may decide to terminate the pregnancy if it is a major abnormality.

If The Test Result Is "Low Risk", Does it Guarantee That The Baby Will Be Healthy?

No. The screening test is not a guarantee that the baby will be healthy. It only helps to screen for Down Syndrome and spina bifida. If the test result is "low risk", it means that the chances of having these conditions are reduced.

Overall, about 6 -7 out of 10 Down Syndrome babies will be detected by the serum screening. However, there will be still be some that are undetected and will be born to mothers who have had a "low-risk" result.

Using a cut-off risk of 1 in 250, the rate of detection among mothers at "high risk" has been shown to vary according to the maternal age as indicated below :

Maternal age Detection rate
Less than 20 years 46 %
Between 20 - 24 years 47 %
Between 25 - 29 years 50 %
Between 30 - 34 years 60 %
Between 35 - 39 years 77 %
Over 40 years 91 %

Do I Need The Test?

The test is entirely voluntary. In deciding whether or not to have the test, you should consider carefully what you and your spouse would do if the test reveals an increased risk of Down Syndrome or other abnormalities.

How Soon Would I Know The Test Result?

Results are usually available within 3 days of the test.

What Should I Do If I Would Like To Go For The Maternal Serum Screening Test?

Please inform your doctors if you wish to have the test. You will then be given an appointment for the blood test between 15 to 20 weeks of your pregnancy based on an ultrasound scan.

A Word Of Reassurance

The great majority of babies are healthy.

Reading the information here may have started you worrying about some possible problems with your pregnancy. Please keep in mind that the great majority of babies are born perfectly healthy. For most women, the test is likely to provide further reassurance that all is well.

If you have any questions about the maternal serum screening test, please do not hesitate to ask our Nurse Counsellors at the Antenatal Monitoring Clinic.


Frequently Asked Questions And Answers For Women With "High Risk" Test Results

What is Down Syndrome?

Down Syndrome, or Mongolism as it was formerly known, is a congenital disease caused by a defect in the chromosomes. Chromosomes are structures in our body cells that carry the genetic make-up passed down from parent to child. The defect usually occurs while the ovum is developing.

In a normal person, there are 23 pairs of chromosomes. A person with Down Syndrome has extra chromosome number 21.

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What Are The Characteristics Of A Down Syndrome Baby?

Babies born with Down Syndrome have a characteristic facial appearance, are mentally handicapped and may also have other birth defects. About half of them will have congenital heart disease.

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Who Are Risk Of Having A Down Syndrome Baby?

Every woman faces a risk of having a baby with Down Syndrome. The risk increases as a woman becomes older.

For example, about 1 in 1,000 women who are 28 years old will give birth to a baby with Down Syndrome, but a woman who is 38 years old has a higher risk of about 1 in 200.

It is not true that the risk of having a Down Syndrome baby is reduced if the mother had given birth to normal children before.

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Are Older Mothers More At Risk Of Having A Down Syndrome Pregnancy?

Older mothers are known to be at a higher risk of carrying a Down Syndrome Baby. Pregnant women over the age of 35 years are therefore usually offered amniocentesis, a specific test to detect Down Syndrome.

Until recently, it was thought that a woman's age was the only way to estimate what her risk of having a baby with Down Syndrome would be. Now we can make use of the levels of AFP and hCG in her blood as well as her age to estimate the level of risk.

With the new blood test, older mothers found to belong to the "low-risk" group do not need to consider amniocentesis.

However, it must be remembered that the blood tests do NOT guarantee that the baby does not have Down Syndrome. A "low risk" result only indicates the chances of having a baby with Down Syndrome is low.

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If The Amnicontesis Test Result Show That My Baby Has Down Syndrome, Can This Be Corrected?

The answer is unfortunately no. Down Syndrome cannot be corrected. However, you may choose to terminate the pregnancy.

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What Is Amniocentesis?

Amniocentesis is a procedure that involves collecting a sample of the amniotic fluid that surrounds the baby in the womb. And ultrasound scan is first done to determine the best site in which the fine needle will be inserted through the abdomen. The skin of the abdomen is first cleaned with an antiseptic before the needle is inserted to collect a small amount of the amniotic fluid.

The amniotic fluid contains cells from the baby that can be used to diagnose Down Syndrome. It is sent to the laboratory and the result is known 3 - 4 weeks later.

No test can guarantee that a baby will be born free of all birth defects but if the result of the amniocentesis is negative, it will rule out Down Syndrome.

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Is Amniocentesis Safe?

Amniocentesis has been carried out for a number of years and every year over 1,000 mothers in KK Women's and Children's Hospital undergo the procedure.

Experts in the field of maternal-foetal medicine have carefully studied its safety both locally and overseas. We know that occasionally it can cause a miscarriage. It is estimated that about 1 in 300 women who have had an amniocentesis will have a miscarriage as a result of the procedure.

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