First trimester biochemistry screening

A screening test has been developed that increases the likelihood of Down syndrome pregnancies being detected in the first trimester by combining the results of a blood test from the mother and ultrasound examination of your baby's neck region. This combined screening test can pick up about 90% of pregnancies affected by Down syndrome.

This ultrasound image shows the fetal neck and the nuchal translucency measurement which is used to help assess the risk for chromosome abnormalities.

A maternal blood sample is taken between 10 and 14 weeks in the first trimester of pregnancy and the amount of two substances called PAPP- A and free beta-hCG are measured. Using ultrasound, the thickness of the fluid under the skin at the back of the baby’s neck is measured. This measurement (nuchal translucency) and the results of the blood test are combined in a calculation to give a risk of your baby having Down syndrome.

If you receive a 'high risk' result your doctor will discuss options with you for further invasive diagnostic testing (see Prenatal diagnosis testing), which checks the chromosomes of your baby. A 'high risk' screening result does not mean that your baby has Down syndrome but that there is an increased risk of Down syndrome. The great majority of prenatal diagnostic test results show normal chromosomes with only a small proportion confirming a chromosome abnormality.

For more information, see Biochemistry testing.