Combined first-trimester screening

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).

Accurate measurements of fetal crown-rump length and nuchal translucency are made during an ultrasound scan, and a maternal venous blood sample is taken. The measurements must be recorded on the test laboratory form that goes with the sample to the biochemistry lab.

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 Patient with higher-chance first-trimester combined screening result.

• Combined first-trimester screening is offered to pregnant women between 11 and 14 weeks’ gestation.
• The screening test combines information from a maternal blood test and an ultrasound scan to determine the chance that the fetus is affected by trisomy 21, 18 or 13.
• Where a higher-chance result is given (over 1 in 150), further assessment should be offered.

For clinicians

• The 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, issue 2, pages 256–268. DOI: 10.1002/uog.15945

For patients

• Antenatal Results & 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