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Serum concentrations of the free beta subunit of h CG are higher than average, and pregnancy-associated protein A concentrations are lower, in the presence of a fetus with Down syndrome.The combination of free beta-h CG, PAPP-A, and maternal age appears to yield detection and false-positive rates comparable to second-trimester serum screening (63 % and 5.5 %, respectively) (ACOG, 2001).

Main outcome measures were detection and false-positive rates for screening with a threshold risk of 1 in 200 at term, and the detection rate achieved for a false-positive rate of 2 %.In October 1999, the ACOG issued a position statement that first trimester screening is investigational and should not be used in routine clinical practice.The ACOG statement concluded that “[f]irst-trimester screening for fetal chromosome, cardiac, and other abnormalities using the nuchal translucency marker alone or in combination with serum markers appears promising but remains investigational.” Based on the results of the FASTER trial, which found that first-trimester screening is as good as or better than second-trimester screening, ACOG (2004) stated that first-trimester screening using nuchal translucency, free beta-h CG, and pregnancy-associated plasma protein-A has comparable detection rates and positive screening rates for Down syndrome as second-trimester screening using 4 serum markers (AFP, beta-h CG, u E3, and inhibin-A).The American College of Obstetricians and Gynecologists stated that, although first-trimester screening for Down syndrome and trisomy 18 is an option, it should be offered only if certain criteria can be met.Wright et al (2010) provided estimates and confidence intervals for the performance (detection and false-positive rates) of screening for Down's syndrome using repeated measures of biochemical markers from first and second trimester maternal serum samples taken from the same woman.

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