In the first-trimester screening test, two biochemical markers are analysed and combined with maternal age and the nuchal translucency measurement to generate a combined result.
The biochemical markers are:
- pregnancy-associated plasma protein-A (PAPP-A); and
- free ꞵ-human chorionic gonadotropin (free ꞵhCG).
An accurate measurement of the fetal crown-rump length and the nuchal translucency is required, as well as a maternal venous blood sample. The measurements must be recorded on the test laboratory form that goes with the sample to the biochemistry lab.
Where a higher-chance result is given (over 1 in 150), further assessment should be offered. This may include further ultrasound imaging in a specialist fetal medicine unit, non-invasive prenatal testing or diagnostic testing requiring chorionic villus sampling or amniocentesis.
For further information, see our In the Clinic scenario, Patient with higher-chance first-trimester combined screening result.
- Fetal Medicine Foundation: Nuchal translucency scan
- Ghi T, Sotiriadis A, Calda P and others. ISUOG practice guidelines: Invasive procedures for prenatal diagnosis. Ultrasound in Obstetrics and Gynaecology 2016: volume 48, pages 256 – 268. doi: 10.1002/uog.15945
- Antenatal Results and Choices: Amniocentesis
- NHS: 12–week scan
- NHS: Screening for Down’s syndrome, Edwards’ syndrome and Patau’s syndrome
- SOFT UK
- Tommy’s: The booking appointment