This prenatal test can help your healthcare practitioner assess your baby’s health.
The NT test uses ultrasound to measure the clear (translucent) space in the tissue at the back of your developing baby’s neck.
The NT scan must be done when you’re between 11 and 14 weeks pregnant. The last day you can have it done is the day you turn 13 weeks and 6 days pregnant. It’s usually offered along with a blood test in what’s known as first-trimester combined screening.
Like other screening tests, an NT scan won’t give you a diagnosis. But it can assess your baby’s risk for certain problems and help you decide whether you want to have chorionic villus sampling (CVS) or amniocentesis to find out whether your baby is actually affected.
What to Expect
What are the advantages of first-trimester screening?
First-trimester screening lets you learn about your baby’s risk for chromosomal problems relatively early in the pregnancy without subjecting yourself to the slight risk of miscarriage from an invasive test like amniocentesis or CVS. If the risk is low, the results will offer you some reassurance.
The NT scan has been performed in the United States since 1995, mostly at large medical centers. Ultrasound technicians (sonographers) and doctors need special training and high-resolution ultrasound equipment to perform it correctly. They must be certified by the Fetal Medicine Foundation in London, the organization that sets the international standards and provides the software that enables a doctor to evaluate your baby’s risk.
Because the NT scan requires special training and equipment, it’s not yet available everywhere. Sand Lake Imaging has several technicians who are trained and certified in this test.
How can I decide which tests are right for me?
The American College of Obstetricians and Gynecologists now recommends that women of all ages be offered first- and second-trimester screening and diagnostic testing options. Your practitioner should discuss the pros and cons of various approaches with you. If you need further information, consider meeting with a genetic counselor. But ultimately, whether to test and which tests to have is a personal decision.
Many women opt for screening first and then make a decision about diagnostic testing based on the initial results. Other women opt for diagnostic testing right away. They may know that they’re at high risk for a chromosomal problem or a problem that can’t be detected by screening. Some women decide to have no screening or testing at all.
How is the NT scan done?
The sonographer first confirms your baby’s gestational age by measuring him/her from crown to rump to see if it’s about the size he should be for his age. Then the sonographer positions the sensor, called the transducer, over your abdomen so that your baby’s nuchal fold area shows up on the monitor and measures the thickness of it on the screen with calipers.
How are the results computed?
Your baby’s chances of having a chromosomal abnormality are determined by the nuchal fold measurement, your age, and your baby’s gestational age. If you’re having a first-trimester combined screening, the blood test results will also be factored in.
Your age is factored in because although anyone can have a baby with a chromosomal abnormality, the risk increases as you age. For example, your likelihood of carrying a baby with Down syndrome ranges from about 1 in 1,295 at age 20 to about 1 in 82 at age 40.
Researchers have measured the nuchal folds of thousands of babies between 11 and 14 weeks of pregnancy. Because a baby’s nuchal fold will normally get a bit thicker with each day of gestation, they’ve been able to establish what an average thickness is for each day during those three weeks.
They’ve also figured out the statistical relationship between this measurement, the baby’s age, the mother’s age, and the likelihood that the baby will be born with certain abnormalities. In general, the thicker the fold at a given gestational age, the higher the chance of a chromosomal problem.
How accurate is this test?
How reliable a test is can be seen in its detection rate. The detection rate tells you how good the test is at identifying babies that are truly affected as being high risk. Specifically, it’s the percentage of affected babies who show screen-positive results, out of all those being screened.
The NT scan alone will detect about 70 to 80 percent of babies with Down syndrome. The detection rate for the first-trimester combined screening (the NT scan plus a first-trimester blood test) ranges from 79 to 90 percent.
What if the results indicate that my baby is at high risk for a problem?
With the help of your practitioner or a genetic counselor, you’ll want to decide whether the results indicate a high enough risk that you want to have amniocentesis or CVS to get a definitive diagnosis.
If you decide not to have diagnostic testing, you can get more information about your baby’s health and development by following up with a detailed ultrasound at 18 to 20 weeks. This ultrasound can detect “soft markers” of chromosome disorders, such as short limbs, a bright dot in the heart, a bright bowel, and certain problems in the kidneys. It can also look for anatomical defects, such as spina bifida.
How to PrepareAbdomen Ultrasound Preparation: Nothing to eat, drink, or smoke 6 hours prior to appointment time. Medications may be taken with a small amount of water.
OB Less than 14 weeks and Pelvic Ultrasound Preparation: 1½ hours before exam time, empty your bladder and drink 32 oz. of water, finishing 1 hour prior to exam. Arrive with a very full bladder. Do not urinate. Pelvic ultrasound should not be scheduled during menstruation.