Nuchal translucency is the area just under the skin at the back of the fetal neck. It is important because if the fetus has a greater-than-normal amount of swelling at the back of the neck, there is a high likelihood that the baby will have Down Syndrome or a major heart problem or both. 
Most of us in the business of prenatal diagnosis believe it is desirable to discover defects before delivery in order to be prepared for any adverse outcome a baby might have. Not all people believe this however.

Nuchal translucency takes its place with the triple screen blood test (a screen for abnormal chromosomes, such as Down Syndrome, and also a screen for neural tube defects such as spina bifida) and prenatal ultrasound anatomic survey as the methods we use to look for abnormalities in the unborn fetus. 

Nuchal translucency must be done between 11 weeks 0 days and 13 weeks 6 days menstrual gestational age. The swelling is transient and may be gone if you look after 14 weeks. This time window in the pregnancy is a little too early to do the anatomical survey, but it is a good time to do not only nuchal translucency, but also date the pregnancy, check for multiple gestations, and to look for gross abnormalities. 

The ideal thing would be to have two ultrasounds during your pregnancy, one around 11 weeks, and another around 20 weeks. It is best to have the nuchal translucency done early in the 3-week window, because it is not always possible to obtain the correct view of the fetus, and if you are early in the window, we can have you return to try again. If you have your nuchal translucency scan at 13 weeks 4 days, and we cannot obtain the view we want, we do not have much time to try again. 

The view has to be right along the middle and length of the fetus. The method of choice is the vaginal approach at this time in pregnancy because we have so much better resolution than through the abdomen. 

So what is one to do if they have to pay for their own prenatal ultrasound, or they have a third party payer that will pay for only one ultrasound in the pregnancy? 

If you are young (under age 35), you would probably be better off getting the ultrasound done at 16-20 weeks. 

If you are 35 or older it’s a personal decision: your risk of having a baby with a chromosomal abnormality (such as Down syndrome) increases with age, so you should try to have both the nuchal translucency testing and the anatomical survey. Babies with abnormal chromosomes have a high incidence of anatomical abnormalities, such as congenital heart disease. 

One thing to consider is that a majority of Down babies are born to low risk young women, because young women have far more babies than older women. So it is very difficult to say that a young woman does not need to have nuchal translucency measured. And most obstetrical providers are obtaining triple screens on all pregnant women regardless of their risk category. The difference is that ultrasounds cost more than the triple screen blood test. 

If your obstetric provider is going to order a first trimester ultrasound, and there is no particular reason why it should be done before 11 weeks, you could ask that you wait until 11 weeks 0 days. You will have not only the benefits of an early ultrasound, but also the nuchal translucency measurement.

Realistic Expectations

Prenatal ultrasound cannot diagnose all problems with a fetus, so you should never interpret a normal prenatal ultrasound exam as a guarantee that you are going to have a normal baby. 

Some abnormalities are very difficult to find, or may not have developed at the time in the pregnancy that you have your scan. The position of the fetus in the uterus has a great deal to do with how well we see structures. You may be asked to return in an hour or two or even a day or two to complete the examination, in the hopes the fetus has moved. 

Images tend to be dramatically clear in thin patients with lots of amniotic fluid and not so clear in heavy women, particularly if there is not much amniotic fluid. 



Excerpt from the Medical Ultrasound Fact Sheet from the American Institute of Ultrasound in Medicine (AIUM)