First Trimester Screen (FTS) - nuchal translucency
The First Trimester Screen (FTS) is a test performed between 12wk4d –13wk4d for any chromosomal abnormality such as Downs Syndrome. The test combines blood test results and a nuchal translucency ultrasound scan, both matched to the patient’s age.
The FTS correlates these findings with a likelihood risk to determine if the patient is at high risk of having a baby with a chromosomal abnormality. The FTS does not directly diagnose an abnormality, but predicts the likelihood of this type of abnormality being present.
Nuchal Translucency Test
The Nuchal Translucency test is part of an obstetric ultrasound examination performed between 12wk4d –13wk4d. Please advise your patient to make an appointment as soon as she receives the referral to ensure she is scanned during that period. Gestational age is calculated from the first day of the last menstrual period (LMP) or from an earlier ultrasound-dating scan. Please ask the patient to have this information available when she phones South Coast Radiology to make a booking.
All fetuses retain a small amount of fluid around the head and neck, called the nuchal translucency, at 12–13+4 weeks’ gestational age. The thickness of the nuchal translucency can be measured during an ultrasound scan. Usually, there is an increase in the nuchal translucency when the foetus has a chromosomal abnormality.
South Coast radiology perform nuchal translucency scans at the following locations:
- John Flynn Private Hospital
- Pindara Private Hospital
- Runaway Bay
What is a FTS risk?
All women regardless of age have a small risk of giving birth to a baby with a chromosomal abnormality. The FTS test assesses whether a patient’s risk is less than, equal to, or greater than the average risk for her age.
Why should a patient have a nuchal translucency scan?
The nuchal translucency test is part of a non-invasive ultrasound study that provides information for doctors about the relative risk of a fetus having chromosomal abnormality. Additional benefits of the ultrasound study include confirmation that a fetus is alive, accurate dating of the pregnancy, early diagnosis of major physical defects and the detection of a multiple pregnancy.
How is the test performed?
First, a transabdominal obstetric ultrasound is performed to assess fetal anatomy. Particular care is taken to measure the nuchal translucency. In approximately 10% of cases a transvaginal (internal) ultrasound scan might be required to gain a more accurate measurement.
Who performs the study?
A specialist sonographer accredited and licensed by the Fetal Medicine Foundation (FMF) in London will perform the ultrasound and a South Coast Radiology radiologist will report on the ultrasound examination, and provide the information needed to determine the individual risk. The sonographer will give the patient an opportunity to ask any questions she might have about the FTS scan.
Pathology blood test
The blood test measures two chemicals – PAPP-A and free ß hCG. These chemicals are produced by the placenta and cross into the bloodstream. In Downs Syndrome pregnancies the levels of these chemicals in the bloodstream can be abnormal. Please advise patients that a pathology company performs the blood test, not South Coast Radiology.
When should I have my blood test?
The patient should have a blood test taken when she is 10 weeks’ gestational age. The fetal gestation age is calculated from the LMP or from an earlier ultrasound-dating scan.
How are the results calculated?
South Coast Radiology uses the FMF software accredited for calculating the risk of Downs Syndrome. The program calculates an individual risk for the pregnancy, taking into account the patient’s age, gestational age, the thickness of the nuchal translucency and blood test results.
When does the patient get the results?
The patient’s ultrasound images and report will be delivered to the referring doctor on the following working day.
How reliable is the test?
FTS is the most accurate first-trimester screening test for chromosomal abnormalities currently available. A low risk cannot exclude Downs Syndrome or other chromosomal disorders, as FTS is not a diagnostic test.
If a patient chooses to have the nuchal translucency alone, the detection rate is approximately 75-80%. If she has the combined FTS the detection rate improves to approximately 90%.
The test can also give a false high-risk score for a baby that does not have a chromosomal abnormality. This false positive result will occur in approximately 5% of pregnancies tested.
The patient will still need to have an 18-20 week morphology ultrasound to assess fetal anatomy and growth, the cervix and placental location.
What do the results mean?
The result only gives an indirect indication of the likelihood of a fetal chromosomal abnormality. A low-risk score does not mean there is no risk, and a high-risk score does not mean that the fetus definitely has a chromosomal abnormality.
To acquire a correct diagnosis of a chromosomal abnormality, a direct diagnostic test such as chorionic villus sampling (CVS) or amniocentesis is needed. Usually the care provider will refer the patient to an obstetrician who specialises in this field.
To complete our records and quality assurance regarding our staff’s technique, and to contribute to the ongoing 11–13+6 week study by the Fetal Medicine Foundation, we need to know the outcome of your patient’s pregnancy. Could you please encourage patients to complete the form given by our staff, at the time of the nuchal translucency scan.
Your cooperation in this matter is greatly appreciated. With your help, we can continue to monitor and improve future information provided for pregnant women.