It is now possible during the early stages of pregnancy to estimate the risk of a mother having a Down’s affected baby using a combination of biochemical tests and ultrasound measurements. These tests may also indicate the possibility of other chromosomal disorders, such as Edward’s syndrome (Trisomy 18) and Patau's syndrome (Trisomy 13). This type of screening provides an opportunity to assess the risk of these conditions being present without performing more invasive procedures such as chorionic villus sampling (CVS) or amniocentesis. Only those women whose screening results indicate that they are in a higher risk group are counselled and offered these further tests.
Screening for Down’s syndrome should ideally be performed before 14 weeks and 2 days, but later screening is possible.
An ultrasound scan is offered at about 11 to 13 weeks of pregnancy. This involves taking a measurement of the nuchal translucency (NT) which is the sonographic appearance of a collection of fluid under the skin behind the fetal neck in the first-trimester of pregnancy. This measurement maybe used on its own, or in combination with biochemical results from a blood test (performed on the mother), to calculate the risk of the baby having Down’s syndrome.
This ‘combined test’, comprising nuchal translucency, beta-human chorionic gonadotrophin, and pregnancy-associated plasma protein-A (PAPP-A), should be offered to screen for Down’s syndrome between 11+2 weeks and 14+1 weeks of pregnancy . The results from these tests are used to calculate the chance of the pregnancy being affected by Down’s syndrome or another chromosome abnormality. For women who attend later in pregnancy, blood tests may be offered between 14+2 and 20 weeks (second trimester maternal serum screen). At this time during pregnancy the blood levels of alpha fetoprotein (AFP), human chorionic gonadotrophin (hCG), unconjugated oestriol (uE3) and Inhibin A (InhA) may be measured. This is commonly referred to as the quad or quadruple test because of the use of four markers.
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 small proportion of women with a result in the higher risk category for Down’s syndrome will 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: Second trimester maternal serum screen, unconjugated estriol