First Trimester Combined Screening

The FTS is done between 11 weeks plus two days and 13 weeks plus three days of pregnancy. You will have both the ultrasound and the blood test on the same day.  

The ultrasound is called a nuchal translucency (NT) ultrasound and the blood test measures the levels of two proteins in your blood. The FTS uses the results of these tests along with your age to determine your results.  

Typically, you will receive the results from your screening test from your health-care provider within seven to 10 days after the test. The results will provide you with a numerical value that reflects the chances that your pregnancy might be affected by Down syndrome, trisomy 13 or trisomy 18. For example, this numerical value could be 1/8 or 1/2000. If your result is greater than 1/350, it will be labelled positive (showing a higher chance of the conditions). If your result is less than or equal to 1/350, it will be labelled negative (showing a lower chance of the conditions).

While more than 99 per cent of pregnancies unaffected by these conditions receive negative results, approximately 10 to 15 per cent of pregnancies with those genetic conditions may also receive a negative result. This means that not all affected pregnancies will be detected by the FTS.  

Your chances of a positive result increase with your age because the chance of having a pregnancy with Down syndrome increases with age. Around four to five per cent of all tests will get a positive result, with the majority being false positives. That means that most of these pregnancies will not be affected by Down syndrome, trisomy 13 or trisomy 18.  

Between 85 and 90 per cent of the pregnancies affected with Down syndrome will screen as positive. If your results are positive, you may choose to have a diagnostic test to find out if your pregnancy truly is affected by those genetic conditions. We offer chorionic villus sampling (CVS) or amniocentesis at Mount Sinai Hospital. 

There are no risks associated with FTS to you or your pregnancy.

The limitations of FTS include the accuracy of the test and what kinds of conditions it can screen for.

FTS cannot provide a definitive diagnosis of genetic conditions. It is a screening test that can tell you how likely it is that your pregnancy is affected by certain genetic conditions. More tests are needed to receive a diagnosis.

FTS has a lower detection rate compared to non-invasive prenatal testing (NIPT), which can be done any time after 9 or ten weeks of pregnancy.  

FTS also has a higher false positive rate compared to NIPT, meaning there is a greater chance of receiving a positive result when the pregnancy is not affected by the conditions screened for.

FTS does not screen for open neural tube defects or other birth defects.

The blood test that is part of the FTS may not provide reliable information for multiple pregnancies (twins or more). However, the NT ultrasound findings can still be used.