Maternal Serum Screening test for Down Syndrome and Neural Tube defects:
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 neural tube defect.
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 amniocenteses ( 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 amniocenteses.
The result of the maternal serum screening test is reported as either "screen- negative" or "screen- positive".
What does "screen- positive" mean?
A woman may be "screen- positive" for two reasons:
* 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 mothers blood is more than 2 and a half times the normal value.
* Screen-positive because of an increased risk of Down's syndrome. This means that the risk of giving birth to a baby 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 385 or more.
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.
What are the further tests?
* an ultrasound "dating" scan to check how far the pregnancy is advanced (unless one has already been done). AFP, uE3 and hCG levels vary as the pregnancy goes along, so that the 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.
* diagnostic tests (amniocenteses and a detailed ultrasound scan) if the screening test is still positive after the "dating" scan.
What does "screen-negative" test results mean?
If the risk of giving birth to a baby with Down's syndrome is found to be less than 1 in 385, and the AFP level is not as high, then the screening result is called "negative-screening". More than 9 out of 10 woman 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
* approximately 3 out of 4 cases of all neural tube defects
What happens if the diagnostic test shows that there is an abnormality?
The results will be discussed with the woman concerned 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
THE TEST MUST BE DONE AFTER 15 WEEKS OF PREGNANCY AND PREFERABLY BY 18 WEEKS. SCREENING IS MOST ACCURATE WHEN DONE BETWEEN 16-18 WEEKS.
IF YOU HAVE TWIN PREGNANCY OR HAVE INSULIN DEPENDANT DIABETES, THIS MUST BE NOTED IN THE INFORMATION SUBMITTED WITH THE TEST
IF AN AMNIOCENTESIS HAS ALREADY BEEN ATTEMPTED IN THIS PREGNANCY, THE TEST CANNOT BE INTERPRETED.
(Information provided by Jennifer Rosendorff PhD, Cytogenetics Laboratory, PO Box 8475, Johannesburg 2000, Tel: 679-5850)