First Trimester or Second Trimester screening or both for Downs Syndrome
Title: First Trimester or Second Trimester screening or both for Down's Syndrome.

Author: FASTER Research Consortium. The members include from the Columbia University College of Physicians and Surgeon, Royal College of Surgeon in Ireland, Boston University, Utah

Introduction: This article is in the New England Journal of Medicine, Nov. 10, 2005 Vol. 353 Number 19. It is not clear what the best way to screen pregnant women for Down’s Syndrome is: to perform First Trimester screening or Second Trimester screening, or both, and incorporate the results. A trial was conducted which is called FASTER (First And Second Trimester Evaluation of Risk) and the goal of this study was to provide direct comparative data on the available screening methods for Down’s Syndrome from a large population. This a large perspective study with level one evidence.

Method: This study was conducted at 15 US centers from October 1999 to December 2002. Institutional review board approval was obtained and written consents were given by participants. Inclusion criteria were a maternal age of 16 years or older. Women with singleton pregnancies were screened for Down’s Syndrome measuring nuchal translucency and PAPP-A and Free beta hcg at 10 3/7 through 13 6/7 wks gestation. Crown rump length of 36mm to 79 mm. Ultrasonography was performed by specially trained ultrasonographers according to a standardized protocol. A minimum of 20minutes was reserved for the assessment and trasvaginal ultrasonography was used if it was necessary. The patient could return for second evaluation if the first one failed. All images were scored by a single reviewer at the main study center and feedback was given to the ultrasonographers. A random selection of 10% of images underwent additional review by an independent ultrasound quality assurance committee. Median nuchal translucency measurements and their standard deviations were monitored according to ultrasonographer and study site. Any differences in these values were reviewed and feed back was given to the ultrasonographers. Women were excluded from the study if they had prior measurement of translucency or if anencephaly was diagnosed. Infants who had septated cystic hygroma were followed separately without contributing serum samples. Second trimester screening was done between 15 & 18 weeks measuring quad screen which include alpha fetal protein, total hcg, unconjugated estriol, and inhibin A. the risks were calculated from the measurement of these markers together with maternal age. A comparison was made by step wise sequential screening (risk results provided after each test), fully integrated screening (single risk result was given), and serum integrated screening similar to fully integrated screening, but without nuchal cord translucency.

Assessment of Risk: measurements of markers were converted into multiples of median for gestational age, adjusted for maternal weight, race and ethnicity. Nuchal translucency, multiples of median values were central specific and the mean of 3 measurement was used for calculation of risk. The risk of Down’s Syndrome was estimated by multiplying the maternal age/specific odds of the live birth of an infant with Down's Syndrome by the likelihood ratio obtained from the overlapping gaussian distributions of affected and unaffected pregnancies.

Results: First trimester screening was performed in 38,167 patients. 117 had a baby with Down’s syndrome, at a 5% false positive rate. The first trimester combined screening detected 87% of Down’s Syndrome at 11 weeks gestation. At 12 weeks, it was 85%, and at 13 weeks, 82%. (note: here we see that at 11 weeks, there is the highest detection rate)
With step wise sequential screening, the detection rate was 95%. With serum integrated screening: 88%, and with fully integrated screening: 96%. First trimester screening was performed at 11 weeks.
Paired comparisons found significant differences between tests except for the comparison between serum integrated and combined screening.

Conclusion: First trimester combined screening at 11 weeks is better than second trimester quadruple screening. But at 13 weeks, the results were similar to second trimester quadruple screening. Step wise sequential screening and fully integrated screening have high rates of detection, with low false positive rates.

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