What is a First Trimester Screening?
A first trimester screening, performed roughly between 11-13 weeks of pregnancy, is used to detect chromosomal abnormalities such as Down's Syndrome, Edwards' Syndrome and Patau's Syndrome. It involves two parts, a blood test and an ultrasound screening referred to as a ‘Nuchal Scan’ or ‘Nuchal Translucency Screening.’ The screening helps your OB/GYN determine the fetus’s risk of these abnormalities.
How is a Nuchal Scan Performed?
A nuchal scan is done by ultrasound. The ultrasound technician will measure the amount of fluid in the back of the baby's neck. While all babies have some fluid in the back of their necks, the presence of excess fluid in the back of a baby's neck can be a symptom of an abnormality.
When Is a Nuchal Scan Performed?
Nuchal scans are generally performed between 11 and 14 weeks gestation. Tests completed before or after this range are not as accurate.
How Accurate are Nuchal Scans?
Nuchal scans are screening tests - not diagnostic tests. This means that they can be used to assess the risk or likelihood that your baby has an abnormality, but they cannot be used to diagnose a condition. If your nuchal scan does detect an abnormality, you will then have the option to pursue further diagnostic testing.
First trimester combined screenings typically detect Down Syndrome 85 percent of the time. However, they also give a false positive 5 percent of the time, meaning that they indicate an abnormality that doesn't exist.
What Should I Do If My Nuchal Scan Comes Back Positive?
If your nuchal scan indicates that there might be an abnormality, the first thing you need to do is try to relax. Just because the scan says something could be wrong with your baby's chromosomes does not necessarily mean that something is.
At this point, you will likely have the option to pursue diagnostic testing such as chorionic villus sampling or amniocentesis for a better understanding of what is going on. These tests do come with increased risks, however, so you will want to talk to your doctor to find out which course of action is best for you.
What Risks are Associated with Nuchal Scans?
Nuchal scans themselves present no known risks. However, receiving a false positive can lead to undue anxiety, more testing and even terminating a pregnancy needlessly.
Nuchal scans are routine and relatively risk-free, so if your doctor recommends one, do not be alarmed. He or she is likely just checking to be sure.