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First-trimester Maternal Serum Screening for Down Syndrome

written by: •edited by: DaniellaNicole•updated: 12/5/2008

Prenatal diagnosis for Down syndrome can be done in the first-trimester and is combined with ultrasonographic nuchal translucency measurement, plasma protein A (PAPP-A) and human chorionic gonadotrophin (free beta-hCG) measurements. All of these are combined to asses the risk for Down syndrome.

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    The clinical standard for prenatal diagnosis of Down syndrome has been to offer maternal serum screening in the 15th to 16th weeks of gestation. However, a new clinical standard for detecting Down syndrome is becoming more widely used. The new standards include the use of first-trimester screening combined with the use of serum screening by associated plasma protein A (PAPP-A), human chorionic gonadotrophin (free beta-hCG) and ultrasound screening for nuchal translucency measurements [1].

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    Human chorionic gonadotrophin (free beta-hCG)

    Free beta-hCG, a hormone that is important for regulating a healthy human pregnancy, is used in maternal serum screening because it can be detected in maternal plasma or urine by the 8th or 9th day after ovulation has started. Free beta-hCG levels can vary throughout pregnancy, reaching a peak level at about 8 to 10 weeks and declining there after. Approximately every two days, during the first four weeks of gestation, the free beta-hCG levels will increase and lower [2]. During the 6th to 7th weeks of gestation, the levels of free beta-hCG can take up to three and a half days to double [3]. Therefore, as the pregnancy progresses after the 12th week of gestation, the free beta-hCG hormone is not being produced as often [4]. Elevated levels of free beta-hCG can help identify Down syndrome during pregnancies [5]. Research is still being done to figure out the specific function of the free beta-hCG hormone.

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    Nuchal translucency screening test

    The nuchal translucency screening test is a prenatal test that allows healthcare practitioners to asses the risk of a baby being born with Down syndrome. The measurement of the nuchal fold is an important indicator for the development of Down syndrome because there is a larger than normal build-up of fluid in the back of the neck. A thickness of between 2.5 to 4 mm, with 3 mm the most commonly used measurement, is used to asses if a fetus is at risk for Down syndrome [6].

    The accuracy of the nuchal fold is very important for risk assessment. The factors can an affect the accuracy of the measurement for the nuchal fold includes: fetal position, and measurement technique [6].

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    Down Syndrome Prognosis

    There is no cure for Down syndrome at this time. It is important to note that a lot of children that are born with Down syndrome live a full life, attend regular schools, attend regular classes, play in special olympic events, and much more.

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    References

    [1]. Evaluation of first trimester maternal serum and ultrasound screening for Down's syndrome in Eastern and Northern Finland. European Journal of Human Genetics (2001) 9, 404-408

    [2]. Chartier M, et al. Measurement of plasma chorionic gonadotropin (hCG) and CG activities in the late luteal phase: Evidence of the occurence of spontaneous menstrual abortions in infertile women, Fertil Steril 31:134,1979.

    [3]. Ashitaka Y, et al. Production and secretion of hCG andhCG subunits by trophoblastic tissue. In Segal S (ed):Chorionic Gonadotropins. New York, Plenum, 1980 p 151.

    [4]. American College of Obstetricians and Gynecologists. Medical Management of Tubal Pregnancy. Practice Bulletin Number 3, December 1998. Washington, D.C. ACOG, 1998

    [5]. Jenn HSU, et al. Urine Free Beta-hCG and Total Estriol for Down Syndrome Screening During the Second Trimester in an Asian Population. Obstetrics and Gynecology 1999, 94: 107-111

    [6]. Taipale Pekka, et al. Increased Nuchal Translucency as a Marker for Fetal Chromosomal Defects. The New England Journal of Medicine. Volume 337: 1654-1658. 1997