Pro & Con: should nuchal translucency measurements be done routinely?

YES

The new standard has shifted from screening for Down syndrome at 16 weeks' gestation to screening at 12 weeks' gestation using nuchal translucency measurements. Nuchal translucency thickness is the best single marker in screening for Down syndrome, and it should be the foundation of any screening that we do.

The first large study involving nuchal translucency measurement as a method of screening for Down syndrome was reported in the Lancet in 1998 (Lancet 352[9125]:343-46). This provided data on 96,127 women and 300 fetuses with Down syndrome. It found a relatively high detection rate that could compete with detection rates from second-trimester biochemical tests.

Measuring first-trimester nuchal translucency is established clinical practice in many centers in the world. Already in 1998, 8% of women in the United Kingdom had nuchal translucency screening available to them. We don't have more recent data on availability.

There are now 16 other large studies conducted in low-risk populations where the effectiveness of nuchal translucency screening can be judged. They were conducted worldwide, which suggests that this technique should work in a heterogeneous population such as that which you have in the United States. Overall, these studies provide us with data on 200,000 pregnancies screened by nuchal translucency measurement, providing a massive consensus of opinion that this is a useful test. Some observers have tried to manipulate the Fetal Medicine Foundation data to say that the detection rate is much lower. The arguments are based on the fact that some affected fetuses die during pregnancy, and therefore earlier screening picks up fetuses that would have died before birth. There are four publications that have dealt with this by developing mathematical models. Essentially, they show that intrauterine lethality decreases the sensitivity of this test by 4%-5%. This means a detection rate of 72%, rather than 77%, but stil l much higher than the 60% inferred by some groups. A recent Australian study suggests that when given the choice, women would prefer an early test.

Although we've reached some consensus about how useful nuchal translucency is to screening, you can see that there is divergence of data, partly because different investigators screen at different gestational ages and partly because of the way people have used different cutoffs for establishing a false-positive group. That's why the Fetal Medicine Foundation has developed a standardized technique and promotes training and testing for those learning this technique to make sure we are all measuring nuchal translucency in the same manner. Training improves detection rates dramatically.

Even the biochemists acknowledge that "the issue is not whether nuchal translucency is useful, but establishing how it should be used in combination with biochemical markers" (J. Ultrasound Med. 21[4]:481, 2002). The future for screening is a first-trimester test that combines ultrasound (nuchal translucency measurement) and biochemistry ([beta]-HCG and pregnancy-associated plasma protein A). Recently a prospective study of 15,000 women reported a 90% detection rate with a 5% false-positive rate. There are data from 10 more studies that support that--while the critics focus on one Scottish study that did not perform as well.