What is being tested?

The maternal serum screening tests involve the measurement of different pregnancy-associated hormones, which are found in all pregnancies.

Several different biochemical substances are measured in the blood. Which substances are measured depends on the time during pregnancy that the sample was taken. The combinations of tests may be known by different names depending usually on how many tests are measured. For example, the first trimester screening uses the concentrations of pregnancy-associated plasma protein A (PAPP-A), the free ß component of human chorionic gonadotrophin (free ß-hCG) combined with ultrasound measured nucal translucency to calculate the probability of Down syndrome. The triple test combines the results of three different substances (AFP, hCG and unconjugated oestriol (UE3); the inclusion of inhibin A in a four-test panel is called the "Quadruple test".

NIPT measures the amount of cell-free DNA circulating in the mother's blood.

First trimester screen:

• pregnancy associated plasma protein A (PAPP-A) is one of several proteins produced by the placenta.  The PAPP-A concentration increases throughout pregnancy in the mother's blood until term and low levels of maternal serumm PAPP-A are associated with fetal Trisomy 21.
• Free beta human chorionic gonadotrophin (free ß-hCG) is a hormone produced by the trophoblast tissue of the placenta. Concentrations of the free ß-hCG increase to a peak at around 8-12 weeks of gestation and then decline to 10% to 15% of the peak concentrations and plateau from 20 weeks until term. Free ß-hCG provides a more sensitive marker of Down syndrome than total hCG.
• Ultrasound measurement of fetal nuchal translucency (NT) or the thickness of the fluid accumulated at the back of the baby's neck. An increased NT is strongly associated with a higher probability of Trisomy 21.

The ultrasound part of this test can only be performed between 10 weeks and 13 weeks 6 days pregnancy. The biochemistry part of the test can be performed from 9 weeks until 13 weeks 6 days of pregnancy.

Second trimester screen, MSS or ‘triple test’:

• Alpha fetoprotein (AFP) is the major blood protein produced in early fetal life and it is excreted into the amniotic fluid by the immature fetal kidneys. AFP passively diffuses into the maternal blood.  
• AFP concentration peaks in fetal blood between 12 and 14 weeks and inmaternal serum between 28 and 32 weeks of gestation. An abnormal increase in maternal serum AFP is the basis of the serum screening test for NTDs at about 15-16 weeks of pregnancy.
• Human chorionic gonadotrophin (hCG) is a hormone produced by the placenta. Concentrations of free ß-hCG provide a more sensitive marker of Down syndrome than total hCG.
• Unconjugated oestriol (uE3) is a hormone produced by placenta from precursor hormones produced by the baby, so it requires both placenta and baby to be healthy and developing normally.

How is it used?

The majority (70%) of screening tests performed in Australia are performed in the first trimester where the results from the biochemical tests and ultrasound in first trimester screening are combined together to calculate the probability of the foetus having Down syndrome.

The screening tests serves only to screen for specific conditions: that is, to identify those pregnancies at increased risk of the more common foetal anomalies such as Trisomy 21 and NTDs, which might benefit from diagnostic testing through chorionic villous sampling (CVS) or amniocentesis.

Interpretation of the screening test results:

First trimester screening

• First trimester concentrations of PAPP-A tend to be lower in pregnancies with Down syndrome.
• Free ß-hCG concentrations tend to be higher in pregnancies with Down syndrome.
• Nuchal translucency is increased in pregnancies with Down syndrome.

Second trimester screening

• The maternal  serum AFP concentrations  tends to be lower than usual in pregnancies with Down syndrome. A fetus with an open neural tube defect has an opening in the spine, head or abdominal wall that allows higher than usual amounts of AFP to pass into the mother’s blood. In this case maternal serum AFP levels will be higher than usual.
• The maternal free ß-hCG concentration tends to be higher than usual in pregnancies with Down syndrome.
• In Down syndrome pregnancies the UE3 concentration tends to be lower than usual.
• The second trimester screen can be performed from 14 weeks for the diagnosis of Down syndrome, but when screen for neural tube defects, maternal serum AFP is best used after 15 weeks gestation.

There are many reasons for increased maternal serum AFP results (false positive values) that are not associated with NTDs or chromosomal abnormalities. The most important are incorrect gestational age and feto-maternal transfusion.

Calculation of the likelihood of a Down syndrome affected pregnancy

A complex mathematical calculation is performed, using the biomarkers measured in the screening tests, together with details of the gestational age, mother's age and weight, to calculate the chance of the baby having Down syndrome ( trisomy 21), or other chromosomal abnormality such as trisomy 18 or 13.

If the probability of detecting a chromosomal or structural abnormality is greater than a certain cut-off value, further diagnostic tests (aminocentesis or CVS) will be offered.

NIPT assesses the risk of the baby being affected by Down syndrome (trisomy 21), Edward syndrome (trisomy 18) and Patau syndrome ( trisomy 13). Some tests can also assess whether abnormalities of the sex chromosomes such as Turner syndrome and Klinefelter syndrome are present. NIPT has a false positive rate of about 0.2% and detection rate greater than 99& for trisomy 21 and 18 and slightly less for trisomy 13.

NIPT can not detect neural tube defect which is not caused by a chromosomal number abnormality.

• In the case of a positive NIPT result, no further decisions should be made without a confirmatory invasive diagnostic test ( amniocentesis or CVS).
• A first trimester ultrasound should be done prior to NIPT. This allows for confirmation of viability, accurate gestational dating, identification of multifetal pregnancies, placental and other abnormalities, which may negate the requirement for an NIPT. NIPT can be performed as early as 10 weeks. For 3-5% of women the test will fail due to very low fetal DNA fractions in maternal blood which could lead to a delay in diagnosis. Factors associated with low fetal DNA include a high body mass index and early gestational age (<10 weeks).

When is it requested?

First trimester screening tests may be performed as early as 9 or 10 weeks of pregnancy or second trimester screening tests around 15 weeks of pregnancy.

What does the result mean?

Only a small proportion of women who are in the 'higher risk' category for Down syndrome, or have a high AFP result, will have an affected baby.

The interpretation of the screening test result should be provided by the midwife, a genetic counsellor or doctor. They will be able to explain the meaning of the result and offer the choices available to the mother. If the result appears to be in the ‘higher risk’ category, more definitive tests are needed to confirm a diagnosis. For possible chromosome abnormalities, an amniocentesis or chorionic villus sample will usually be needed. Other disorders may be detectable using a detailed ultrasound scan.

Is there anything else I should know?

The first and second trimester screening results are very dependent on an accurate age for the baby and this is why a dating ultrasound scan is recommended prior to screening. If the gestational age of the baby is not correct, the screening result may be falsely high or low. An early ultrasound scan will show the possibility of twins or triplets and these will result in high maternal AFP levels. Before having an NIPT, an ultrasound scan can determine the gestation and confirm the viability of the foetus.

The probability of having a baby with Down syndrome increases significantly with a woman's age such that the chance at age 30 is 1 in 890 and by age 35 years, it is 1 in 355 and 1 in 35 at 44 years of age. Most (66%) foetuses with Down syndrome occur in women under 35. Over the age of 35, about 40% of women will be in the ‘higher risk’ group after screening, just because their age-related risk is so high. However, it is important to stress that most women in the ‘higher risk’ group using biochemical testing will NOT have an affected baby.

Common questions

• What is Down syndrome?

Down syndrome is produced by having an abnormal number of chromosomes and is sometimes called Trisomy 21.  Most children with Down syndrome have some retardation of growth and intellectual development. The risk of Down syndrome increases with the mother's age, especially in women over 40 years old. Edward syndrome or Trisomy 18 is a condition in which there are three copies of chromosome 18. Trisomy 18 is associated with multiple abnormalities and is usually fatal before or shortly after birth.

• What is a neural tube defect?

Neural tube defects are serious birth defects: the brain, spinal cord, or their coverings do not develop completely. There are 3 kinds of neural tube defects:

• Anencephaly : incomplete development of the brain and the skull.
• Encephalocoele: a hole in the skull through which brain tissue protrudes.
• Spina bifida: the most common neural tube defect, in which the spine does not close properly during early pregnancy. (For more information on spina bifida, visit the Better Health Channel)

• What can I do during pregnancy to prevent formation of a neural tube defect?

There is strong evidence to show that taking 0.4 mg of folic acid a day before pregnancy and in the early stages of pregnancy can significantly reduce the risk of neural tube defects.

• Is maternal screening covered by Medicare?

The biochemical blood tests do attract a Medicare rebate. However the newer fetal DNA-based tests for NIPT are not currently covered by Medicare.

More information

Healthdirect Australia: Down syndrome