You have been advised to have a blood test called the Maternal Serum Screen (the other names are Bart’s test, Triple screen, Maternal Serum Test), to evaluate your risk of having a baby with chromosome abnormality or neural tube defect.

WHAT IS A CHROMOSOMAL ABNORMALITY?

Chromosomes are tiny parts of cells that contain our "genes" which are the internal instructions that determine development. There are 46 chromosomes in every cell. Any change in the number or the structure of the chromosomes may result in an adverse effect on development. An example is Down Syndrome (previously called ‘Mongolism’). Individuals with Down Syndrome have 47 chromosomes. The extra chromosome causes developmental delay, intellectual handicap, characteristic facial appearance, and medical problems such as heart defects.

WHAT WILL BE TESTED IN MY BLOOD?

A sample of your blood will be taken between 15 and 17 completed weeks (from your last menstrual period) of pregnancy. Markers, which are substances (proteins) produced by the developing baby and placenta, are measured in your blood and compared to the normal or "average" levels of other women at the same stage in pregnancy. Three markers are tested. These have been found to be different in pregnancies where the baby has Down Syndrome. Your age and the gestation (number of weeks) of the pregnancy are also part of the mathematical formula used to calculate the ‘risk’. The chance of an "increased risk" result is more likely if you are over 37 years old.

It has been shown that when an unborn baby has a chromosome abnormality, such as Down Syndrome, there is a tendency for the levels of the markers in the mother’s blood to be different (some higher, some lower) than usually found. Interpretation of your blood test results may indicate that you are at an increased risk for a chromosomal abnormality, in particular Down Syndrome, in your pregnancy.

WHAT MAY CAUSE ABNORMAL LEVELS IN MY BLOOD?

There are several possible explanations for the different levels :

• Your dates may be incorrect : The levels of the markers in your blood are dependent on the age of the baby. If your pregnancy is not as far as was thought when your blood was taken, then the levels of the markers would be less than the average.

• Normal variation between individuals : Each of the markers can vary among pregnant women, the scientist will see a range of levels from low to high. The differences from the "usual" levels seen in your result may simply be part of the range of normal values.

• The baby may have a chromosome abnormality : This test result may be an indication of a chromosome abnormality in the baby. A sample of the baby’s cells is required to be sure.

• The baby may have a neural tube defect : If the test result shows the level of one of the proteins, alphafetoprotein (AFP) is increased, it may indicate the baby has a neural tube defect. The two most common neural tube defects are spina bifida and anencephaly. (See separate information sheet for explanation of increased AFP)

WHAT DOES "INCREASED RISK" MEAN?

The risk of having a baby with a chromosomal abnormality such as Down Syndrome increases as a woman gets older. When the Maternal Serum Test result indicates an "increased risk", it means that your chances or risk of having a baby with Down Syndrome or another type of chromosome abnormality is higher than 1 in 250. This risk is similar to or greater than of a woman who is at least 37 years old.

Approximately 1 in 20 women (5%) who are under 37 years of age receive an increased result of which indicates that the risk for a chromosome abnormality is higher than average. The NSW data suggests that one woman in 67 with an increased result will have an abnormal baby. That is, the test will draw our attention to the abnormal baby.

WHAT DOES "LOW RISK" MEAN?

One in 1300 ‘normal’ (ie. low risk) results will not show an increased risk when there is an abnormal baby, that is the result will be falsely reassuring. This is sometimes referred to as a false negative.

CAN I GET MORE INFORMATION TO BE SURE ?

To provide more specific and diagnostic information about your pregnancy and the baby, certain procedures will be discussed with you by your doctor. You may then choose to be referred for further tests. To be sure if the baby has a chromosome abnormality or not, the lab needs to look at the baby’s cells. This is usually done by Amniocentesis. Under ultrasound guidance, a needle is inserted into the amniotic fluid and a small quantity of fluid is withdrawn from around the baby. This fluid contains some of the baby’s cells which are then grown in the lab to determine if the chromosomes are normal. This test has a risk of causing a miscarriage (1 in 250 babies). Sometimes blood from both parents also needs to be tested.

WHAT IF AN ABNORMALITY IS IDENTIFIED BY FOLLOW-UP TESTING?

Any woman who has an Amniocentesis procedure faces the possibility that an abnormality will be detected and a choice have to be made about the pregnancy.

If there is an abnormality diagnosed in your unborn baby, you and your partner can receive genetic counselling from a Specialist about the specific diagnosis. You will be given information about the type of abnormality detected and how it may effect the baby. All your options will be discussed. Any decision about the pregnancy should be an informed choice.

For further information on the types of increased riks - chose one of these options :

This information sheet has been prepared by Dr Wendy L. Cox as an aid to your understanding of the test.