What is it? And Where Does it Fit in Prenatal Sonography?
Nuchal translucency is the swelling 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 before birth the defects that 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.
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, see if there are twins, and look for gross abnormalities.
If money were no problem, the ideal thing would be to have two ultrasounds during your pregnancy, one around 11 weeks, and another around 22 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. We can try to do it through the abdomen or through the vagina. I prefer the vaginal approach at this time in pregnancy because we have so much better resolution than through the abdomen. This is particularly true if the patient is on the heavy side. But we have very little room to maneuver the transducer through the vagina, and if the baby is not in just the correct position, we will then try from 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 20-22 weeks. If you are 35 or older I cannot tell you what would be best: your risk of having a baby with a chromosomal abnormality (such as Down syndrome) increases with your age, so you really ought 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.
The Standard of Care in the United States is becoming the first trimester screening for Down and other chromosomal abnormalities. This screening consists of an ultrasound measurement of the fetal nuchal translucency, plus a maternal blood test to measure the levels of substances in the blood called PAPP-A and free beta HCG.