First Trimester Genetic Screening

 First Trimester Genetic Screening (FTGS) is the combination of a blood test and an ultrasound that measures the fluid around the baby’s neck (nuchal translucency). It is used to identify fetuses that might be at an increased risk for certain birth defects and chromosomal abnormalities, specifically Down Syndrome (Trisomy 21) and Trisomy 18.

FTGS is being offered to pregnant women of all ages between 11 and 14 weeks of pregnancy. It is combined with the mother’s age to give a better estimate of the risk of Down Syndrome (Trisomy 21), as compared to the mother’s age-related risk.  For example, if the age-related risk is assigned at 1 in 300 and the patient has FTGS performed, the assessed risk may be calculated at 1 in 1,500. This represents a significant reduction of the risk of Down Syndrome (Trisomy 21) for increased reassurance, but not diagnostic of Trisomy 21, or screening for other chromosomal abnormalities. 

Women that are 35 and over are traditionally offered diagnostic testing such as CVS (Chorionic Villus Sampling) or Amniocentesis. The age of 35 was selected because the risk of a woman having a baby with Down Syndrome was equal to the risk of prenatal loss following a diagnostic procedure, about 1 in 220. Many women that are age 35 ad over are not comfortable with the risk of fetal loss due to a diagnostic procedure. They choose FTGS to determine their screening risk because it is non-invasive, without risk to the fetus, and done before the second trimester. 

Studies in the United States confirm that the detection rate for Down Syndrome with FTGS is 75 to 80 percent with a false positive rate of 3 to 5 percent. The European studies quote a higher detection rate of approximately 90 percent.  In women 35 or over the detection rate is close to 87 to 90 percent with a false positive rate of 5 percent.

A FTGS result that shows an increased risk does not mean that the fetus has Down Syndrome or Trisomy 18 since this test is on-diagnostic. The patient should be offered further information (such as genetic counseling) ad diagnostic testing (CVS or amniocetesis).

It takes approximately one to two days to receive the results of FTGS. The results are sent to DVIF& G, who will then notify the patient.

It is recommended that women having FTGS done should not have a second trimester quad screen done, as this will not significantly improve the detection of Down Sydrome. It is recommended that an AFP (alpha feta protei) test be given at 15 to 22 weeks of gestation to detect spina bifida ad other birth defects that will not be detected by FTGS.

By Laurie Miller, Staff Sonographer for DVIF& G

Date Posted: February 13, 2007