First Trimester Combined Screening
First Trimester Combined Screening (FTS) is a screening test for chromosome conditions and birth defects. With FTS, two measurements are used, together with your age, to estimate your chance of having a baby with Down syndrome, trisomy 18, or trisomy 13. The first measurement is the nuchal translucency (NT) measurement, taken by ultrasound, and the second is a blood test.
The Nuchal Translucency (NT) Measurement
The NT measurement involves a special ultrasound done between 11 weeks and 13 weeks 6 days of pregnancy. A measurement of the fluid at the back of the baby's neck (called nuchal translucency) is taken. All babies will have some fluid here. The fluid is more likely to be increased in pregnancies with Down syndrome.
The Maternal Blood Test
The maternal blood test measures two substances (PAPP-A and free Beta-hCG), which are normally found in the blood of all pregnant women. In pregnancies with Down syndrome, the levels of free Beta-hCG and PAPP-A tend to be out of the expected normal range. FTS has an 82 percent detection rate for Down syndrome. This means that 82 percent of pregnancies with Down syndrome will be identified at increased risk with FTS.
What type of results should I expect from FTS?
You will be given a personal risk estimate that tells you how likely it is that your pregnancy will be affected with Down syndrome, trisomy 13, or trisomy 18. In the FTS report, this is called the "adjusted risk."
What happens after FTS?
Your FTS result can be used to decide about other prenatal testing options (such as amniocentesis). Women who receive a risk result that places them at an increased risk for Down syndrome, trisomy 18, or trisomy 13 are offered the option of having a diagnostic test such as amniocentesis or CVS. Regardless of a woman's age or FTS results, the decision to have a diagnostic test is a personal one.
All women should be offered an ultrasound 18 to 20 weeks into their pregnancy.
Benefits of FTS
Early, more accurate screening gives peace of mind to many women. The FTS personal risk estimate can be used to assist women in making a more informed choice about diagnostic testing. Some major birth defects can be detected during the NT scan, but not all.
Limitations of FTS
About 1 in 20 women will receive an increased-risk result. It is normal to be worried when you hear you are at an increased risk. Most women with an increased risk result do have healthy babies. An increased-risk result does not mean that the baby has a chromosome condition. A reduced-risk result does not guarantee a healthy baby.