First trimester ultrasound performed between 11 and 14 weeks is an important early assessment of your pregnancy. This scan can be used to accurately date your pregnancy, to evaluate abnormalities in your ovaries and cervix, to accurately determine chorionicity in twins, to assess for placental abnormalities and to perform a nuchal translucency screen. In addition, at San Diego Perinatal Center, with our high resolution equipment, we are able to detect 40% of major fetal anomalies such as neural tube defects, abdominal wall defects, renal, limb and cardiac abnormalities on this early scan.
Nuchal translucency/First trimester screening – this blood test screens for Down Syndrome (Trisomy 21) and Trisomy 18. It is part of the California Prenatal Screening Program and is a free service to all pregnant women. Additional testing and counseling would be recommended if the test is positive. This test has about an 85 % detection rate for Down Syndrome.
Nuchal Translucency Screening for Detecting Trisomy 21 (Down Syndrome) and Trisomy 18.
During development of the embryo, there is a clear space beneath the skin at the back of the neck which we call the nuchal translucency. Between 11 and 14 weeks of pregnancy, this space can be measured by ultrasound and the size of this space is associated with the risk for fetal chromosome abnormalities and the risk for fetal heart defects. We use this measurement as an early screening tool to identify pregnancies at higher risk for these conditions.
If we can accurately measure the nuchal translucency, then a computer program can be used to calculate the risk for Down Syndrome and Trisomy 18 using the mother’s age and blood hormone markers. Women who screen positive at increased risk would then be offered further testing. The nuchal translucency screen will identify 85-90 percent of Down syndrome cases and 80-85 percent of Trisomy 18 cases. Approximately 5-10% of the time, we are unable to obtain this measurement either because the baby does not cooperate or there are other technical factors like abdominal wall scarring, fibroids or maternal obesity.
In the State of California, there is a prenatal screening program which is used for risk calculations. The state requires that all first and second trimester blood specimens are performed in the state laboratory system. https://www.cdph.ca.gov/Programs/CFH/DGDS/Pages/pns/
It is important to realize that the only way to be certain that a baby does not have a numeric chromosome abnormality like Down syndrome is by means of Amniocentesis or Chorionic Villous sampling.
Risk for Other Birth Defects
The nuchal translucency measurement is more than just a screening for Down syndrome. A very small nuchal translucency measurement – less than 2.5 mm – places the pregnancy in a low-risk group for problems, such as fetal heart abnormalities. Pregnancies where the nuchal translucency measurement is between about 2.5 and 3.5 mm are in an intermediate-risk group, and for these women, a genetic sonogram at about 18 weeks into the pregnancy plus a detailed sonogram of the heart (called a fetal echocardiogram) at about 22 weeks are recommended. Women with a nuchal translucency measurement of 3.5 mm or more have pregnancies at increased risk for chromosome problems, for abnormalities of the fetal heart, and for other birth defects, ranging from abnormalities of the skeletal system to the baby’s diaphragm. For these pregnancies, genetic counseling and a genetic sonogram at 18 weeks, and a fetal echocardiogram and a detailed scan at about 22 weeks, are recommended.
In the past, sonography at 11 to 14 weeks was not an effective way to screen for birth defects of the baby. The embryo at that stage was just too small to evaluate well. As time has gone on, the ultrasound equipment and our ability to interpret the images have both improved. Now, at the 11 to 14-week scan, we are able to identify many major central nervous system, cardiac, abdominal, and limb abnormalities. Our center’s goal is to identify 40 percent of major structural malformations at the time of the nuchal translucency scan.
When is the best time to schedule a nuchal translucency scan?
The accuracy is highest when the lab work is done between 10 and 11 weeks into the pregnancy, and we recommend scheduling the nuchal scan between 12 and 13 weeks. As long as the lab test has been done more than a week before, we should be able to provide you with the Down syndrome and trisomy 18 risk calculations at the time of your ultrasound scan.