Every pregnancy has a risk that the fetus carries a serious genetic defect. Doctors have developed ways to calculate the risk of a serious chromosomal abnormality fairly early in pregnancy. In the first trimester, nuchal translucency, or the degree of fluid pooling at the back of the fetus' neck, combined with tests of blood levels of specific proteins and hormones, allows doctors to calculate the likelihood that the fetus suffers from a serious chromosomal defect such as Down syndrome.
Definition
Nuchal translucency, also known as a nuchal fold scan, refers to the ability to see through the area at the back of the neck of the fetus using ultrasound. In a normal fetus, a dark space doesn't exist between the skin and bones at the back of the neck. In a fetus with nuchal translucency, fluid accumulates in the space, creating a dark shadow. As Dr. Nicolaides points out in his 2004 article in "American Journal of Obstetrics and Gynecology," it is not the presence of translucency itself but the actual thickness of the translucent area that determines the risk that the fetus has a genetic abnormality.
Detection
The presence and thickness of nuchal translucency can be measured by ultrasound in the first trimester of pregnancy, between 11 to 14 weeks. A skilled ultrasound technician must perform the test to ensure accurate measurements, because factors such as fetal position can significantly alter the results. According to Dr. Nicolaides, skilled ultrasound technicians successfully measure nuchal translucency by abdominal ultrasound in 95% of cases.
Significance
A thick area of nuchal translucency indicates that fluid had pooled between the skin and neck bones of the fetus. Although no one is sure of the chain of events connecting a defect in the number of chromosomes to fluid pooling at the back of the neck, thick nuchal translucency strongly associates with serious genetic disorders such as Down syndrome. Research by prenatal surgery expert Dr. Kypros Nicolaides' shows that about 75 percent of fetuses with Down syndrome have increased thickness of nuchal translucency. A formula that encompasses the thickness of nuchal translucency, the week of pregnancy and the mother's age determines the likelihood that the fetus has a genetic defect.
Follow-Up
Thick nuchal translucency indicates a potential genetic problem and most doctors will suggest that the pregnant woman should follow up the finding with more specific and accurate genetic testing, such as amniocentesis or chorionic villus sampling. Research by obstetrician and gynecologist Christine Comstock and her colleagues published in a 2006 article in "American Journal of Obstetrics and Gynecology" reveals that nuchal thickness of 4mm or more, regardless of the age of the fetus, indicates such a high risk of a chromosomal defect that a doctor might counsel the woman to opt for tests immediately.
Considerations
In "Obstetrics: Normal and Problem Pregnancies," Dr. Steven Gabbe says that in 5 percent of cases of increased thickness of nuchal translucency, the fetus has a normal number of chromosomes. However, in those cases, the fetuses still had higher rates of other defects and death as compared to fetuses with normal nuchal translucency.
References
- American Journal of Obstetrics and Gynecology; "Nuchal translucency and other first-trimester sonographic markers of chromosomal abnormalities"; Kypros Nicolaides; 2004
- American Journal of Obstetrics and Gynecology; "Is there a nuchal translucency millimeter measurement beyond which there is no added benefit from forst trimester serum screening?"; Christine Comstock et al.; 2006
- "Obstetrics: Normal and Problem Pregnancies"; Steven Gabbe, Jennifer Niebyl and Joe Leigh Simpson; 2007
