Down syndrome occurs when a fetus carries three copies of chromosome 21 instead of two copies. Only a test of the fetus' genes can prove if a fetus has Down syndrome, but ultrasound can detect certain characteristic traits associated with it. A 3D ultrasound might be slightly better at detecting Down syndrome, saving some women from undergoing amniocentesis, the definitive but invasive test for Down syndrome. However 2D ultrasound is nearly as effective as 3D ultrasound, and unlike 3D ultrasound, 2D ultrasound is routinely available.
Process
In a standard ultrasound test, a technician runs a hand-held transducer over the abdomen to send sound waves under the skin. The sound waves bounce off of the fetus and return to a computer that converts them into a two-dimensional image. In 3D ultrasound, the computer takes thousands of images at once, and layers and shades them to produce an image with depth and much greater clarity.
Availability
As of 2010, the technology of 3D ultrasound is still fairly new and not widely available. It is not used routinely and will not be offered to most pregnant women. If there is a nearby facility and Down syndrome is suspected, a doctor might refer a patient for 3D ultrasound if she feels that 2D will not give a definitive answer.
Uses
According to "Pregnancy for Dummies," no medical evidence proves that 3D ultrasound does a better job of detecting fetal abnormalities than the standard 2D tests. However, 3D ultrasound can give better detail of the face, feet and hands of the fetus, areas that can reveal an increased risk that the baby has Down syndrome. Some doctors feel that 3D ultrasound provides clearer results when measuring nuchal translucency, the volume of fluid collecting at the back of the neck, another indicator of Down syndrome. With 3D ultrasound, the doctor might be able to easily distinguish the nuchal membrane from the amniotic membrane to get a more precise measurement.
Signs
A 3D ultrasound can reveal signs of Down syndrome such as increased nuchal translucency, short arms along with a thick neck, and lack of a bone in the nose. According to Luis Goncalves, M.D., and his colleagues in their 2004 article in "Journal of Ultrasound Medicine," using 2D ultrasound it can be difficult to tell if the nasal bone is missing, or if the bone has not yet formed over the cartilage. The clear picture provided by 3D ultrasound makes it much easier to judge the presence or absence of the nasal bone.
Effectiveness
According to "Pregnancy for Dummies," the combination of the triple or quadruple blood test for Down syndrome with standard 2D ultrasound detects 80 to 90 percent of all cases of Down syndrome. Using 3D ultrasound to look for the absence of the nasal bone, Dr. Goncalves and his colleagues were able to correctly identify 90 percent of cases of Down syndrome, and "Your Pregnancy Week by Week" cites it as being 95 percent effective in detecting Down syndrome.
References
- American College of Obstetricians and Gynecologists: About Ultrasound
- "Pregnancy for Dummies"; Joanne Stone, Keith Eddleman and Mary Duenwald; 2009.
- "Your Pregnancy Week by Week"; Glade Curtis and Judith Schuler; 2008.
- "Journal of Ultrasound Medicine"; Luis Goncalves et al.; Phenotypic characteristics of absent and hypoplastic nasal bone in fetuses with Down syndrome; 2004.
