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Down Syndrome Birth Rate Lower After Screening

In Denmark, first-trimester screening seemed to lower the number of prenatal diagnostic procedures and halve the number of infants born with Down syndrome.

In 2004, the Danish National Board of Health recommended that pregnant women be offered first-trimester Down syndrome risk assessment based on maternal age, nuchal translucency, and a double test consisting of serum-free β-human chorionic gonadotropin and pregnancy-associated plasma protein A (similar first-trimester screening is recommended in the U.S. [JW Womens Health Mar 8 2007]). Chorionic villus sampling or amniocentesis was recommended for women with calculated risks above 1:250 to 1:400 (cutoffs varied by county).

Now, analysis of Danish national health data from 2000 through 2006 shows that the proportion of Down syndrome cases diagnosed prenatally rose from 53% to 79% and the number of invasive diagnostic procedures was halved during this period. Screening performance in 2005 and 2006 was evaluated. In 2005, 63% of pregnant women underwent risk assessments and 4% screened positive (1:300 risk); 81% of these women proceeded to diagnostic procedures. In 2006, 84% of pregnant women underwent risk assessments and 3.5% screened positive, of whom 90% underwent diagnostic procedures. False-negative rates were 0.04% in 2005 and 0.01% in 2006; false-positive rates ranged from 4% in 2005 to 3% in 2006. From 2000 through 2006, the Down syndrome birth rate fell substantially (55–65 yearly for 2000 through 2004; 31–32 yearly for 2005 through 2006).

Comment: Presumably the decline in Down syndrome birth rates observed in this national study occurred because many women who received prenatal diagnoses chose to have abortions. The authors did not explain why 10% to 20% of women with positive screens declined to undergo diagnostic procedures — although some women likely did not want the procedures because of risk for miscarriage (albeit very low) or for religious or personal reasons. In obtaining informed consent for first-trimester screening, healthcare providers should emphasize that the test is a risk assessment and therefore neither establishes nor rules out a diagnosis of Down syndrome. In addition, clinicians should be aware that some women might decline to be screened because they would refuse diagnostic testing or therapeutic abortions in any case.

— Wendy S. Biggs, MD

Published in Journal Watch Women's Health December 30, 2008

Citation(s):

Ekelund CK et al. Impact of a new national screening policy for Down’s syndrome in Denmark: Population based cohort study. BMJ 2008 Nov 27; 337:a2547. (http://dx.doi.org/10.1136/bmj.a2547)

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