First Trimester Combined Screening (FTS)
This screening test combines measurements from a nuchal translucency ultrasound and a blood test, typically done on the same day, to estimate the chances of a baby having Down syndrome.
The FTS takes into account the measurement of the ultrasound and the level of the proteins measured in the blood test. It then adjusts for the woman’s age and gives a numerical estimate of the chance of Down syndrome, such as 1/2000 or 1/8.
The ultrasound and blood tests are done between 11 weeks, 2 days and 13 weeks, 3 days of pregnancy, and the results of the FTS are sent to your caregiver (obstetrician, family doctor or midwife) in 7 - 10 days.
Results are categorized as either screen positive or screen negative.
If the test indicates the chance for Down syndrome is greater than 1/350, the result is called screen positive. The test will also indicate a screen positive for Trisomy 18/13 if the chance of chromosome abnormalities Trisomy 18 and Trisomy 13 is greater than 1/100.
Approximately 4 - 5% of women taking this test will screen positive, but the majority of these women will NOT be carrying a pregnancy affected with Down syndrome (also known as a false positive result). Only a small proportion will be affected with Down syndrome (a true positive).
The chance a woman will screen positive also increases with her age, because the rate of Down syndrome pregnancies increases as a woman ages.
The FTS has close to an 85 - 90% detection rate for Down syndrome, meaning 85-90% of the pregnancies affected with Down syndrome will screen positive.
A less than 1/350 chance of Down syndrome is a screen negative result, and more than 99% of pregnancies WITHOUT Down syndrome will screen negative.
However, between 10-15% of pregnancies with Down syndrome will also screen negative (false negative), meaning results are not absolutely guaranteed.
Advantage of FTS
The main advantage of the FTS is speed, with results of both the ultrasound and screening forwarded directly to your health care provider within 7-10 days. This allows for earlier diagnostic testing (CVS) and decision making.
Disadvantage of FTS
A disadvantage of FTS in the general literature is that it is less accurate than an integrated prenatal screening (IPS) which also screens for Down syndrome. 10-15% of pregnancies with Down syndrome screen negative with an FTS, compared to 10% with IPS.
The false positive rate is also higher with FTS at 4-5%, compared to 2-3% with IPS.
In addition, another blood test (AFP) is required in the 2nd trimester to determine an increased chance of Spina Bifida.