It is now possible during the early stages of pregnancy to estimate the possibility of a mother having a Down syndrome affected baby using biochemical tests or genetic tests, usually in combination with ultrasound measurements. These tests may also indicate the possibility of other chromosome disorders, such as Edward’s syndrome, or an open neural tube defect (NTD), such as spina bifida. This type of screening provides an opportunity to assess the possibility of these conditions being present without performing more invasive procedures such as chorionic villus sampling (CVS) or amniocentesis.
At some time after 9 weeks of pregnancy, the mother may be given the option of having a screening test for Down syndrome. At this time there are two options. Measurement of blood levels of pregnancy associated plasma protein A (PAPP-A) and human chorionic gonadotrophin (hCG) may be made. The results from these tests will be used to calculate the chance of the pregnancy being affected by Down syndrome or another chromosome abnormality. The biochemical results at this early time in pregnancy will not give an indication of an NTD. This conventional testing attracts a Medicare rebate. There is also a newer option called Non-Invasive Prenatal Testing (NIPT) of cell-free fetal DNA (cffDNA). This test uses a blood sample taken from the mother’s arm and the test measures free DNA from the fetus that is circulating in the mother’s blood. Early results from this new test indicate that it is likely to be both more sensitive and more specific than the current first or second trimester biochemical tests. This type of DNA-based testing is not covered by a Medicare rebate and currently costs several hundred dollars. For more information see our April 2015 News item.
The biochemical results are usually combined with the results of an ultrasound scan performed at about 10 to 13 weeks of pregnancy. This is known as combined first trimester screening. The ultrasound involves taking a measurement of the thickness of the skin and tissue at the back of the baby’s neck. Called the nuchal translucency (NT), this measurement will be used, usually in combination with the biochemical results, to calculate the chance of the baby having Down syndrome.
Later in pregnancy, between 15 and 22 weeks, it is also possible to assess the likelihood of a Down syndrome affected pregnancy using biochemical tests. At this time during pregnancy the blood levels of alpha fetoprotein (AFP), human chorionic gonadotrophin (hCG) and unconjugated oestriol (uE3) may be measured. This is commonly referred to as the triple test because of the use of three markers. The results from these tests will be used to calculate the chance of the pregnancy being affected by Down syndrome or possibly another chromosome abnormality or an open neural tube defect (NTD).
In some hospitals the results from the early testing and the later testing may be combined to give a more comprehensive assessment of the chance of an affected pregnancy. Different testing strategies may be called by various names such as ‘contingency testing’ or ‘the integrated test’.
All the test results are interpreted based on the age of the mother, her weight and other factors such as family history and smoking, to assess the risk of the baby having a chromosomal abnormality. Only a very small proportion of women with a result in the higher risk category for Down syndrome, or an elevated AFP, have an affected baby.
Limitations of screening
Screening tests are not conclusive for the presence of a birth defect but indicate the possibility of a problem. Further tests must be performed to confirm the baby’s condition.
Related tests: Maternal screening, hCG, unconjugated oestriol