First-Trimester Aneuploidy Screening
Down syndrome is the most common genetic cause of mental retardation. The majority of children with Down syndrome are born to mothers over the age of 35, so the prevalence of Down syndrome is increasing as more women elect to delay childbearing. In 2002, the birth prevalence of Down syndrome reached 1 in 629 births.
New screening tests
The most recent development in prenatal screening for Down syndrome has been the introduction of new sonographic and maternal serum-marker tests performed in the first-trimester. Prior to first-trimester screening, the standard for Down syndrome screening was the “quad” screen. It combines the patient’s age along with four maternal serum markers (hence the name “quad” screen). This screening test grew out of the work of Merkatz and colleagues, who noted decreased levels of maternal serum alpha-fetoprotein (MSAFP) in fetal Down syndrome. (1) The quad screen is traditionally performed at 14 to 21 weeks’ gestation and detects Down syndrome with 65 to 80 percent accuracy. As with any screening test, the possibility of a false-positive (and to a lesser degree false-negative) result exists. With an 80 percent detection rate, the false-positive (or screen-positive) rate is approximately 5 percent. Compared with second-trimester Down syndrome screening, first-trimester screening offers the advantage of earlier diagnosis that may reduce the health risks associated with pregnancy termination. An earlier diagnosis may also reduce maternal anxiety and offer greater privacy.
Fetal nuchal translucency
The fetal nuchal translucency (NT) constitutes the ultrasound portion of the first-trimester screen. It refers to the normal subcutaneous fluid-filled space behind the fetal neck. In a large multicenter trial that screened 96.127 patients, Snijders and colleagues validated NT as a screen for aneuploidy in the general population. (2) In this study, the detection rate for trisomy 21 was 82 percent. Since then, other studies (including prospective trials) have confirmed the clinical utility of nuchal translucency to screen for aneuploidy in the general population. (3, 4) The American College of Obstetricians and Gynecologists January 2007 Practice Bulletin entitled “Screening for Fetal Chromosomal Abnormalities” notes that first-trimester screening is effective and comparable to the quad screen. (5) This bulletin emphasizes that all women, regardless of age, should be offered aneuploidy screening.
Pregnancy-associated plasma protein A (PAPP-A) and beta-human chorionic gonadotropin (β-hCG) are the two maternal serum analytes measured in first-trimester screen. These measurements, along with the NT measurement and maternal age, are combined to provide an overall risk assessment for both trisomy 21 and 18. This mode of screening is often referred to as the “combined” test.
Trisomy 18 is a lethal aneuploidy that rises with increasing maternal age. In trisomy 21, PAPP-A decreases by approximately 60 percent and β-hCG increases approximately two-fold. These two serum markers are independent of each other (and of maternal age), and both are independent of the NT measurement. The hCG component of the combined test usually includes the free β-subunit, although the total hCG can be measured with little or no decrease in performance.
Combined Tests
Two large clinical studies have supported the performance of the combined test. The Blood, Ultrasound, Nuchal (BUN) study is a NIH-supported clinical trial that looked at 8,500 high-risk women and demonstrated a Down syndrome-detection rate of 79 percent for a 5 percent false-positive rate. (6) Another large clinical trial known as the OSCAR (One-Stop Clinical Assessment of Risk) study performed in the United Kingdom studied over 12,000 pregnancies and showed a Down syndrome detection rate of 90 percent for a 5 percent false-positive rate. (7)
Two other large prospective studies compared first and second-trimester (quad) screening for Down syndrome. (3, 4) The FASTER (First and Second-Trimester Evaluation of Risk) trial is an NIH multicenter trial that studied over 36,000 pregnancies and showed an 85 percent and 81 percent detection rate for first- and second-trimester screening, respectively, for a 5 percent false-positive rate. Interestingly, another report from the FASTER trial demonstrated an increase in adverse pregnancy outcomes with a low PAPP-A. The Serum, Urine, Ultrasound Study (SURUSS) performed in the United Kingdom with 48,000 pregnancies showed an 86 percent and 81 percent detection rate for first- and second-trimester screening, respectively, again with a 5 percent false-positive rate.
Nuchal translucency associated with heart defects
In the euploid (chromosomally-normal) fetus, an increased NT is associated with an increased risk for congenital heart defects, as well as a myriad of genetic syndromes. (8, 9) Combining the data from six studies with 3,900 pregnancies, the prevalence of cardiac defects was approximately 41 per 1,000 – significantly increased from the baseline risk of congenital heart defects in the general population (8 per 1,000). This prevalence increased as the NT thickness increased. In fetuses with an NT measurement of 2.5 to 3.4mm, the prevalence was 17 per 1,000, and in those with a NT of ≥ 3.5mm, the prevalence was 75 per 1,000. The FASTER trial also confirmed the association between an increasing NT thickness and the risk of congenital heart disease. However, NT should not be used specifically with the purpose of screening for congenital heart defects as it has a low sensitivity and positive predictive value when used in this setting. (9) The association between an increased nuchal translucency and cardiac defects does not have a clear etiology. Transient cardiac failure in the first trimester, as well as abnormal lymphatic development, has been proposed. With regards to congenital anomalies, data combined from 28 studies looking at over 6,100 chromosomally normal fetuses with an increased nuchal translucency, the prevalence of major structural defects was 7.3 percent.
NT screening is performed when the crown-rump ranges from 45 to 84 mm, which roughly corresponds to a gestational age between 11 and 13 6/7th weeks. NT normally increases by approximately 17 percent each week during this time; a measurement greater than the 95th percentile is often used to define an increased NT. A lab or institution may choose to select a specific NT measurement, such that the blood portion of the screen is not obtained, because the NT measurement is high enough to yield an overall positive result. This NT measurement is usually in the range of 3 to 3.5 mm. In twin gestations, the combined test has been validated and thus is appropriate to screen for aneuploidy. For higher-order multiples, usually NT alone without blood analysis is performed.
Tests incorporate first- and second-trimester screening
More recently, new aneuploidy screening paradigms have been developed that incorporate both first- and second-trimester screening. The terminology includes integrated, sequential and contingent screening. Integrated screening involves obtaining NT and PAPP-A in the first-trimester with the patient returning in the second-trimester for a quad screen. These six Down syndrome markers are used to modify the patient’s age-associated a priori risk and provide an individualized risk assessment for Down syndrome and trisomy 18. Importantly, the patient does not receive a risk assessment after the first-trimester component is completed. This non-disclosure has been a criticism of the integrated screening paradigm, because patients may not be willing to wait until the second-trimester for the overall result. However, a significant advantage of integrated screening is its high sensitivity, with low false-positive rate. The detection rate is estimated to be at 90 percent for a two percent false-positive rate. This reduction in the false-positive rate may be attractive to patients, providers and third-party payers in that fewer patients would be exposed to the risks of invasive diagnostic testing (amniocentesis or chorionic villus sampling). (10)
Sequential and contingent screening protocols combine first- and second-trimester screening and allow for an earlier diagnosis, but the disadvantage of non-disclosure of first-trimester results is avoided. In the sequential approach, first-trimester NT and serum biochemistry are performed to yield an interim risk assessment. If the risk is high enough, the patient falls into a screen-positive category and is immediately informed and offered early diagnostic testing. If the initial risk falls below a predetermined cut-off, the patient goes on to the second stage (second-trimester screen) and then receives an overall risk assessment that incorporates both stages of the screen. This overall risk will be reported either as screen-positive or screen-negative. In the latter, the patient’s aneuploidy screening is deemed complete and no further testing is required.
Similar to the sequential screen, the contingent screen also yields an initial risk assessment (communicated to the patient) after the first-trimester component is completed. The difference between contingency and sequential screening is that a screen-negative result can be obtained after the first-stage without requiring the patient to have the second-trimester screen. The purpose of this initial screen-negative category is to eliminate costs and patient anxiety by removing a significant portion (60 to 70 percent) of patients from the pool of those requiring further testing. Similar to sequential screening, a screen-positive result may also be obtained initially and diagnostic testing offered. An “intermediate” risk is another possible outcome. In this scenario, the patient would return for second-trimester screening and an overall risk assessment determined. There are no clear recommendations what cut-offs should be used to define a screen-negative or screen-positive result. A risk of < 1 in 1000 to 1 in 2000 and > 1 in 25 to 1 in 100 may be used, respectively.
There are many screening paradigms that currently exist to screen for aneuploidy, and in particular, fetal Down syndrome. Rather than focusing on the patient at advanced maternal age (≥ 35 years of age at the time of delivery), ACOG recommends that all women should be offered aneuploidy screening regardless of age. Compared with quad screening, the first-trimester combined test offers a higher detection and earlier diagnosis. Strategies that combine first and second-trimester screenings are now available that offer high detection and low false-positive rates.
References:
1. Merkatz IR, Nitowsky HM, Macri JN, et al. An association between low maternal serum alpha-fetoprotein and fetal chromosomal abnormalities. Am J Obstet Gynecol. 1984;148:886-894.
2. Snijders R, Noble P, Sebire N, et al. UK multicentre project on assessment of risk of trisomy 21 by maternal age and fetal nuchal translucency thickness at 10-14 weeks of gestation. Lancet. 1998;351:343-346.
3. Malone F, Canick J, Ball R, et al. First-trimester or second-trimester screening, or both, for Down’s syndrome. N Engl J Med. 2005;353:2001-2011.
4. Wald N, Walter J, Rodeck C, et al. First and second-trimester antenatal screening for Down syndrome: the results of the Serum, Urine and Ultrasound Screening Study (SURUSS). Int J Gynecol Obstet. 2004;111:521-531.
5. ACOG Practice Bulletin. Screening for fetal chromosomal abnormalities. Number 77; January 2007.
6. Wapner R, Thom E, Simpson JL, et al. First-trimester screening for trisomy 21 and 18. N Engl J Med. 2003;349:1405-1413.
7. Spencer K, Spencer CE, Power M, et al. Screening for chromosomal abnormalities in the first trimester using ultrasound and maternal serum biochemistry in a one-stop clinic: a review of three years prospective experience. Br J Obstet Gynaecol. 2003;110:281-286.
8. Hyett, J, Perdu M, Sharland G, et al. Using fetal nuchal translucency to screen for major congenital heart defects at 10-14 weeks of gestation: population based cohort study. BMJ. 1999;318:81-85.
9. Simpson L, Malone F, Bianchi D, et al. Nuchal translucency and the risk of congenital heart disease. Am J Obstet Gynecol. 2007;109(2 part1):376-383.
10. Saller DN, Canick J. Current Methods of Prenatal Screening for Down Syndrome and Other Fetal Abnormalities. Clin Obstet Gynecol. 2008;51:24-36.
