This is a blood test to measure the levels of three substances in a pregnant woman’s blood. The substances are alpha-fetoprotein (AFP), unconjugated oestriol (uE3) and human chorionic gonadotrophin (hCG). The test is usually carried out at about 16 weeks of pregnancy. 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. The test does not detect the abnormalities directly but is a pointer to possible further investigations.

The maternal serum screening test identifies women with an increased risk of Down’s syndrome or neural tube defects so they can decide whether or not to have the necessary diagnostic tests. The diagnosis of Down’s syndrome requires an amniocentesis (a simple procedure to sample the fluid around the baby) and the diagnosis of neural tube defects is made by carrying out a detailed ultrasound scan as well as an amniocentesis.

The result of the maternal serum screening test is reported as either “screen-negative” or "screen-positive”.

What are the reasons for measuring these three substances In the mother’s blood?

We know that women with low AFP in their blood may have a higher than average risk of having a baby with Down’s syndrome. It appears that babies with Down’s syndrome tend to produce less AFP than normal babies and so less is transferred to the mother.

Levels of uE3 in the blood tend to be lower and levels of hCG tend to be higher when Down’s syndrome is present. The reason for this is not known for sure but it may be because Down’s syndrome babies mature more slowly. Levels of these two substances are associated with foetal maturity.

What does a "screen-positive" test result mean?

A woman may be “screen-positive” for a number of reasons:

1.     Screen-positive because of an increased risk of a neural tube defect. This means that the risk of having a pregnancy with a neural tube defect is increased due to the fact that the level of AFP in the mother’s blood is more than two and a half times the normal value.

2.     Screen-positive because of an increased risk of Down’s syndrome. This means that the risk of having a pregnancy with Down’s syndrome, taking into account the woman’s age as well as her blood levels of AFP, uE3 and hCG, is 1 in 250 or more.

3.     Normal baby, but positive result, so called false positive. This may be due to the statistical nature of the test or for another reason such as incorrect dates/length of pregnancy, diabetes mellitus or a twin pregnancy.

Most women with “screen-positive” test results will have normal healthy babies. A screen positive test result does not mean that there is an abnormality. It only means that there is an increased risk and further tests are indicated.

Does a positive test mean my baby is abnormal?

NO! A positive test does not mean that the baby has Down’s syndrome; it only means that there is an increased risk. Five per cent of women may have positive test in the absence of any abnormality in the developing baby. Nearly all women with positive screening tests will have normal babies. It simply means that your risk of having a baby with Down’s syndrome is 1 in 250 or more and you should be offered one or two further tests.

What are the further tests?

The further tests are:

1.     An ultrasound “dating” scan to check how far the pregnancy is advanced (unless one has already been done). AFP, uE3 and hCG levels in the blood vary as the pregnancy goes along, so that levels which may be high or low for an 18 week pregnancy, would be normal for a 15 week pregnancy. If the ultrasound “dating” scan shows that the pregnancy is not as far (or is further) advanced than would be expected by dates (timed from the date of the last menstrual period), then the screening test will need to be re-interpreted and may no longer be regarded as positive.

2.     Diagnostic tests (amniocentesis and a detailed ultrasound scan) if the screening test is still positive after the “dating” scan.

What happens if the diagnostic test shows that there is an abnormality?

The results will be discussed with you and information will be provided about the type of defect that has been found and how it may affect the infant. The possibility of termination of pregnancy will be discussed.

What is my next step if I have already had an ultrasound and it showed that my dates are correct?

You will be offered an amniocentesis to find out whether or not your baby has Down’s syndrome.

Amniocentesis is a simple procedure which involves collecting a sample of the fluid that surrounds the developing baby. An ultrasound scan is done to determine the best site for the amniocentesis and then the skin of the abdomen is cleansed with an antiseptic solution and numbed with a local anaesthetic. A thin needle is inserted through the abdomen and a small amount of the fluid (amniotic fluid) is removed. This fluid contains cells shed from the baby which can be used to make a diagnosis of Down’s syndrome. The fluid is sent to the laboratory and the diagnosis, based on analysis of chromosomes, usually takes about 3 to 4 weeks.

No test can guarantee that your baby will be free of all birth defects but if the result of the amniocentesis is negative it will almost certainly rule out Down’s syndrome.

Is amniocentesis safe?

Amniocentesis has been carried out for a number of years and every year over 15,000 women in Britain have the procedure. Its safety has been carefully studied. We know that occasionally it can cause a miscarriage. It is estimated that about 1 in 200 women who have an amniocentesis will have a miscarriage as a result of the procedure.

If testing shows that my baby has Down’s syndrome can this be corrected?

No. Unfortunately this cannot be corrected, but if you wanted, it would be possible to have a termination of the pregnancy.

What does a “screen-negative” test result mean?

If the risk of Down’s syndrome is found to be less than 1 in 250, 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 negative result.

Does a “screen-negative” test completely exclude Down’s syndrome and neural tube defects?

No, some cases are not detected. The screening test will detect:

·       2 out of 3 cases of Down’s syndrome

·       almost all cases of anencephaly

·       4 out of 5 cases of spina bifida.

Please note:

·       The test must be done after 15 weeks of pregnancy and before 23 weeks to give a valid answer. Clearly the early it is done the better, should an abnormality be found.

·       If an amniocentesis has already been attempted in this pregnancy, the test cannot be interpreted.

·       If you have a twin pregnancy or have insulin dependent diabetes, please telephone for further information as these can affect the test.

·       Patients at Northwick Park Hospital have the test as part of their routine ante-natal care there. This includes an ultrasound scan at the booking visit to accurately date the pregnancy and a blood test between 15 and 17 weeks of pregnancy.

Remember

Although a positive screening test indicates a higher than average risk of Down’s syndrome most women with positive screening tests will have healthy, normal babies.

Nuchal translucency scans for the detection of Down’s syndrome

By carefully measuring the width of the developing spinal cord in early pregnancy, doctors have discovered that it is possible to predict an increased risk of a Down’s syndrome baby. This technique is still at the stage of being a research tool rather than routine NHS practice but can be performed by special request at University College Hospital, London. The scan must be done between 11-13 weeks so needs to be arranged in good time. See your doctor to discuss this if you feel this is necessary.