Here's a case study: Jo, 38 years old, is 11 weeks past the first day of her last period. She read that there was a noninvasive way of detecting her risk of having a baby with some of the most common problems related to advanced maternal age. Her care provider offered to refer her for a nuchal translucency ultrasound after explaining that there were risks that this procedure would not be able to detect. Jo took the test, and the perinatologist reviewed the ultrasound with her, explaining that the result measures 3.4 mm, which corresponds to a very low risk of chromosomal defects.
In fetuses with increased nuchal translucency (NT), the risk of an adverse outcome, which includes chromosomal and other abnormalities, fetal death and death during the first 30 days of life, increases with nuchal translucency thickness from approximately:
- 5 percent for NT of 3.4 mm or less to
- 30 percent for NT of 3.5 to 4.4 mm to
- 50 percent for NT of 4.5 to 5.4 mm and
- 80 percent for NT of more than 5.5 mm
The test is administered between 11 weeks and 13 weeks, 6 days of gestation because:
- It provides women with affected fetuses the option of an earlier and safer form of termination.
- The incidence of abnormal accumulation of nuchal fluid in chromosomally abnormal fetuses is lower at 14 to 18 weeks of gestation than at 11 to 13 weeks of gestation.
- The success rate for taking a measurement at 10 to 13 weeks of gestation is 98 to 100 percent. It falls to 90 percent at 14 weeks of gestation because the fetus is often in a vertical position, which makes it more difficult to obtain the appropriate image.