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First Trimester Screen, hCG

First Trimester Screen, hCG

Test Summary

First Trimester Screen, hCG

  

Clinical Use

  • Prenatal (first trimester) risk assessment for Down syndrome and trisomy 18

Clinical Background

Prenatal testing for Down syndrome and trisomy 18 risk assessment is routinely offered to pregnant women in the second trimester of pregnancy. First-trimester screening, however, is advantageous in that 1) positive results can lead to earlier diagnosis and 2) should results of confirmatory testing indicate the fetus has a chromosomal abnormality, there is reduced maternal morbidity if pregnancy termination is elected. The first trimester screen detection rate, at a constant false-positive rate, is higher than that of the triple screen and the same as that of the quad screen performed in the second trimester.1 The American College of Obstetricians and Gynecologists (ACOG) thus considers first-trimester screening a reasonable option.1

This first-trimester maternal serum screening test includes maternal age, pregnancy-associated plasma protein-A (PAPP-A), total human chorionic gonadotropin (hCG), and nuchal translucency (NT) provided by a certified ultrasonographer. PAPP-A is a placental protein generally present in lower concentrations in Down syndrome-affected pregnancies relative to unaffected pregnancies. hCG is produced by the placenta and levels tend to be increased in Down syndrome-affected pregnancies. NT is an ultrasonographic measurement of a fluid-filled space at the back of the fetal neck. NT tends to be elevated in cases of fetal aneuploidy (eg, Down syndrome).

Individuals Suitable for Testing

  • Women in their first trimester of pregnancy

Method

  • PAPP-A and hCG: immunochemiluminometric assay (ICMA)

  • The multiple of the median (MoM) is calculated for PAPP-A, hCG, and NT.

  • PAPP-A and hCG MoM values are adjusted for maternal weight.

  • Down syndrome risk is based on all 3 MoM values, combined with the maternal age at time of delivery. Expected detection rate is 81% to 83% at a 5% false-positive rate.2,3

  • Trisomy 18 risk is based on maternal age, PAPP-A, and NT. Expected detection rate is 75% at a 0.5% false-positive rate.4

  • If the NT measurement is not provided, the Down syndrome risk will be based on age, PAPP-A, and hCG, and the trisomy 18 risk will be based on age and PAPP-A.

  • Neural tube defect (NTD) risk is not provided. For NTD risk, order a second-trimester (16-18 weeks’ gestation) AFP test.

  • Aliases: Maternal Serum Screen, 1st Trimester

Interpretive Information

Women with a risk <1:270 (Down syndrome) or <1:100 (trisomy 18) are considered at low risk of carrying an affected fetus. Since this is a screening test, such risks cannot guarantee the birth of an unaffected baby.

Women with a risk 1:270 (Down syndrome) or 1:100 (trisomy 18) are considered at increased risk of carrying an affected fetus. Genetic counseling and possible diagnostic testing are recommended.

Inaccurate gestational age and NT measurements can substantially impact risk assessment. Ultrasound confirmation of gestational age is strongly suggested. Risks can be recalculated if necessary; call 866-GENE-INFO.

References

  1. ACOG Practice Bulletin No. 77: screening for fetal chromosomal abnormalities. Obstet Gynecol. 2007;109:217-227.

  2. Palomaki GE, Lambert-Messerlian GM, Canick JA. A summary analysis of Down syndrome markers in the late first trimester. Adv Clin Chem. 2007;43:177-210.

  3. Canick JA, Lambert-Messerlian GM, Palomaki GE, et al; First and Second Trimester Evaluation of Risk (FASTER) Trial Research Consortium. Comparison of serum markers in first-trimester Down syndrome screening. Obstet Gynecol. 2006;108:1192-1199.

  4. Tul N, Spencer K, Noble P, et al. Screening for trisomy 18 by fetal nuchal translucency and maternal serum free β-hCG and PAPP-A at 10-14 weeks of gestation. Prenat Diagn. 1999;19:1035-1042.
     

This test is performed using a kit that has not been approved or cleared by the FDA. The analytical performance characteristics of this test have been determined by Quest Diagnostics Nichols Institute. This test should not be used for diagnosis without confirmation by other medically established means.

Content reviewed 06/2013
 
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