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


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.

We need to support clinicians as they learn to use this test, providing high-quality training and ensuring that quality assurance is placed at the top of the agenda.

Dr. John Hyett is a spokesperson for the Fetal Medicine Foundation and a lecturer at University College, both in London.


Nuchal translucency is the best marker for Down syndrome. Can we recommend it for every pregnancy at this time? We don't have the infrastructure in place to do that.

Many centers are currently using nuchal translucency measurements m combination with serum marker screening in the first trimester. That should be continued, but it is not ready to be offered to every pregnant patient. An editorial in the British Journal of Obstetrics and Gynaecology last year said that only 8% of women in the United Kingdom had access to nuchal translucency screening.

The largest sample set reported by the Fetal Medicine Foundation in 1998 in the Lancet included 96,127 patients screened between 10 and 14 weeks' gestation. A false-positive rate of 5% yielded a detection rate for Down syndrome of 77%. An accompanying editorial suggested that there was under-ascertainment of Down syndrome at the time of screening, and said that if the authors had considered the natural intrauterine mortality of Down syndrome, the true detection rate would have been closer to 60%. To my knowledge, the authors have never satisfactorily answered this criticism.

The Fetal Medicine Foundation data have varied somewhat over the last 6 years, from a false-positive rate of 6% in 1996 with an 84% detection rate, to detection rates between 73% and 79% from 1997 to 2002, accepting a false-positive rate of 5%.

Among the major studies by Fetal Medicine Foundation affiliates, one from the United States reported in 2002 involved more than 8,000 patients screened between 10 and 14 weeks using a combination of nuchal translucency and maternal age. There was a 72% detection rate for Down syndrome with a 5% false-positive rate.

The most recent large study published in 2002, involved approximately 17,000 patients scanned between 10 and 14 weeks in Scotland. The results were very disappointing. Only 52% of patients had three nuchal translucency measurements performed successfully even with training and oversight by the Fetal Medicine Foundation. The detection rate for Down syndrome was 54% with a 5% false-positive rate.

Other recent studies put detection rates at 85%, 72%, or 77%.

With such variation--between 54% and 90%--how can we make a health policy change from a current detection rate with second-trimester serum screening between 60% and 65% for a 5% false-positive rate?

The U.S. standard of care is second-trimester multiple marker screening. With quadruple marker screening, which is now being used more routinely this gives a detection rate of 70%-75% for a 5% false-positive rate.

We also must think about the cutoffs for nuchal translucency and what curves we should use for interpretation. The most recent data from Scotland lead us to believe that generic population medians may not be valid to interpret nuchal translucency The range of multiples of the median experienced in the Scottish trial would produce a fivefold difference in a woman's risk for Down syndrome, depending on which cutoff was used.

Results should be ready from the First and Second Trimester Evaluation of Risk (FASTER) trial in 2004, and a "state of the science" meeting is scheduled for that spring that will address the implementation issues. Until then, nuchal translucency screening is not ready for the general population.

Dr. Mary E. D'Alton is director of maternal and fetal medicine at Sloane Hospital for Women, New York.
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Publication:OB GYN News
Date:Jan 15, 2003
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