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Earlier Screening for Fetal Chromosomal Anomalies

First-trimester screening identified approximately 85% of trisomy 21 cases and 91% of trisomy 18 cases.

In this NIH-sponsored study conducted at 12 U.S. centers, women with pregnancies between 74 and 97 days' gestation were offered screening for trisomies 21 and 18; screening involved risk estimates based on maternal age, blood levels of pregnancy-associated plasma protein A and free ß-human chorionic gonadotropin, and sonographic measurement of nuchal translucency. Fetal chromosomal status was determined by prenatal karyotype analysis if invasive testing was performed or by phenotype evaluation in live births. A total of 8216 women without histories of trisomy 21 or 18 pregnancies were included in the analysis.

Overall, 61 cases of trisomy 21 (prevalence, 1 in 135 pregnancies) were identified: 5 resulted in spontaneous abortions, 15 were electively terminated before week 15, 25 were terminated at week 15 or later, and 16 resulted in live births. In addition, 11 cases of trisomy 18 (1 in 747) were identified.

Using risk-based cutoff values comparable to those currently used for second-trimester screening, early screening identified 85.2% of trisomy 21 cases (false-positive rate, 9.4%) and 90.9% of trisomy 18 cases (false-positive rate, 2.0%). Among women aged 35 or older, 89.8% of fetuses with trisomy 21 and 100% with trisomy 18 were identified. Among the 9 patients with false-negative first-trimester tests for trisomy 21, 7 underwent second-trimester screening; results were positive in 6.

Comment: Compared with second-trimester screening for chromosomal abnormalities, first-trimester screening offers couples greater privacy and earlier information. Women with positive screening results can undergo chorionic villus sampling (CVS) to confirm the results, rather than wait to undergo amniocentesis later in pregnancy -- thus affording the option of earlier, safer termination of pregnancies. However, fewer than half of the women who carried fetuses with trisomy 21 and who chose to terminate their pregnancies did so before 15 weeks' gestation. This observation suggests that limited access to both CVS and abortion might reduce the benefits associated with earlier screening. In addition, some experts express concerns regarding standard measurement of fetal-nuchal translucency. In selected centers, first-trimester screening now can be implemented. However, editorialists suggest that second-trimester screening remain the standard of care, pending results from other studies and revision of practice guidelines.

— Andrew M. Kaunitz, MD

Published in Journal Watch Women's Health December 30, 2003

Citation(s):

Wapner R et al. First-trimester screening for trisomies 21 and 18. N Engl J Med 2003 Oct 9; 349:1405-13.

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• Medline abstract (Free)

Mennuti MT and Driscoll DA. Screening for Down's syndrome -- Too many choices? N Engl J Med 2003 Oct 9; 349:1471-3.

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• Medline abstract (Free)