Combined first-trimester screening
Combined first-trimester screening provides information regarding the probability of a pregnancy being affected by Down syndrome, Edwards syndrome or Patau syndrome. It is offered to all pregnant women between 11 and 14 weeks’ gestation.
Overview
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.
Test outcomes
If the screening returns a low-chance result for Down syndrome, Edwards syndrome and Patau syndrome, no further action is required.
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 Presentation: Patient with higher-chance first-trimester combined screening result.
Resources
For clinicians
- Fetal Medicine Foundation: Nuchal translucency scan
References:
- 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
For patients
- 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