The use of nuchal translucency measurement and second trimester biochemical markers in screening for Down's Syndrome

Abstract

Objective To assess the effectiveness of antenatal screening for trisomy 21 by first trimester sonography followed by second trimester biochemical screening. Design Retrospective five-year review.Setting Maternity unit of a university hospital.Population An unselected group of 7447 pregnant women who had a first trimester scan and nuchal translucency measurement in our unit after January 1995 and had an estimated date of delivery before 1 January 2000. 11.9% were $ 37 years old. A subgroup (n ¼ 4864) also had second trimester biochemical testing by alphafetoprotein and free b-human chorionic gonadotrophin. Main outcome measures Prenatal and postnatal diagnosis of trisomy 21.Results There were 23 fetuses affected with trisomy 21. The overall prenatal detection rate was 87% (20/23; 95% CI 66% to 97%) and we performed invasive procedures in 8.5% of our population. First trimester sonography identified 74% (95% CI 51.6% to 89.8%) of affected fetuses. Second trimester biochemical screening detected half of the fetuses with trisomy 21 which were missed by first trimester screening, increasing the sensitivity to 90.5% (19/21; 95% CI 69.6% to 98.8%) for an invasive procedure rate of 4.2% performed in screened positive women. However, the positive predictive value of the biochemical test was very low (0.5%). In screen negative women, karyotyping for advanced maternal age did not detect any affected fetuses. Conclusion First trimester nuchal translucency measurement is an effective screening test for the prenatal detection of fetuses with Down's Syndrome. Although the measurement of biochemical markers in the second trimester can detect additional affected fetuses this may be outweighed by the delay in diagnosis, the extra visits and cost so that the right time for biochemical screening is most likely to be in the first trimester.

Impact of introducing a national policy for prenatal Down syndrome screening on the diagnostic invasive procedure rate in England

“…Initially there were concerns that when the combined screening test was introduced as a national strategy there would be an increase in the number of chorionic villus sampling procedures performed and that first-trimester invasive procedure services would need to be reorganized in order to ensure that the demand could be met. However, this does not seem to have transpired in practice 11 . The decrease observed in the number of invasive procedures is similar to that found in the Danish population 12 when the combined screening test was first introduced.…”

A model for disclosing the first trimester part of an integrated Down's syndrome screening test

“…Another trend which seems to be gaining in popularity in recent years is sequential NT and mid-gestation serum markers screening (11,(21)(22)(23)(24). Sequential screening means that patients identified as being at an increased risk by the first test will be advised to undergo karyotyping, while the others will be referred to a second test whose results will further determine the action to be taken.…”

Impact of fetal echocardiography

“…Some data does not suggest as strong a relationship of NT to congenital heart disease as the original data of Hyett et al . [91213] A recent multicenter study suggested that around one-quarter of chromosomally normal fetuses with congenital heart disease have an NT value above 3.5mm. [14] This has led to early identification of CHD[14] and an increased demand for early fetal echocardiography on the basis of such findings.…”