The first trimester screening test is one of several different types of screening tests available for estimating the risk of a fetal Down syndrome or Trisomy 18. This screening test consists of a special ultrasound examination of the fetus and a maternal blood test performed between 11 weeks 0 days and 13 weeks 6 days of pregnancy. During the ultrasound exam, the gestational age of the fetus is determined and the nuchal translucency is measured.
The nuchal translucency is an area of skin and fluid at the back of the fetal neck measured with ultrasound that, on average, is larger in fetuses with Down syndrome as well as some other abnormalities. This ultrasound is different from the usual exam because the person doing the exam must be specially trained and certified to perform the nuchal translucency measurement according to strict guidelines.
The blood test involves measuring the levels of two hormones — Pregnancy-Associated Plasma Protein-A (PAPP-A) and human chorionic gonadotrophin (hCG). The results of the blood tests and the nuchal translucency measurement, combined with the mother's age, allow each woman's risk of having a baby with Down syndrome or Trisomy 18 to be estimated.
First trimester screening offers patients the earliest possible assessment of risk for Down syndrome. Patients who receive a screen positive result can make earlier decisions regarding diagnostic testing.
Approximately 85% of women with Down syndrome fetuses and 90-90% with Trisomy 18 fetuses will have a screen positive test result indicating increased risk and recommending a diagnostic procedure (CVS or amniocentesis). For every 100 women who have this screening test, approximately 5-7% will be determined to be at increased risk. However, for every 25-30 women determined to be at increased risk, only one will have a fetus with a chromosome abnormality.
Note: First trimester screening does not screen for neural tube defects.
We have established a clinic to make first trimester screening available to patients. Click here to learn more.