Norton et al claim that sequential screening detects more fetal chromosomal abnormalities than cell-free DNA (cfDNA) analysis. Their study design had significant ascertainment biases that favored sequential screening over cfDNA screening.

Based on the maternal ages in the screened population, the authors calculated elsewhere that the expected number of live births with Down syndrome should have been 1197, but recorded cases equated only to 951. If cfDNA screening had been provided, it is likely that many of the 246 missing cases would have been identified. Moreover, although both sequential and cfDNA study arms included nonviable pregnancies, these were systematically under-counted in their cfDNA analysis. cfDNA would be a replacement for the first-trimester component of a sequential screen; therefore, more nonviable cases would be identified by cfDNA. Based on the theoretic number of 1197 Down syndrome live births and on first- and second-trimester loss rates of 45% and 23%, respectively, 622 losses were expected between the first and second trimester. Additional nonviable Trisomy 18, 13, and monosomy–X pregnancies would also be identified. Ascertainment bias also applied to sex chromosome abnormalities. Norton et al based their analysis on 36 reported cases of XXY, 18 XYY, and 20 XXX. However, the expected numbers of cases were 285, 222, and 207, respectively, and most of these missing sex chromosome abnormalities cases would have been identified through cfDNA screening.

Sequential screening included an unstated number of cytogenetic abnormalities in women with positive neural tube defect screening results; these were omitted in the cfDNA arm, even though these women also received neural tube defect screening. Sequential screening included nuchal translucency >3.5 mm and second trimester Smith-Lemli-Opitz syndrome screening (not widely available outside California). Many of these cases would come to attention after ultrasound examinations without formal nuchal translucency measurements. Sequential screening also included serendipitous detection of balanced translocations and inversions at the time of cytogenetic analysis; these are of only secondary importance.

After accounting for all of these biases, it is clear that cfDNA would have detected substantially more aneuploidies than sequential screening, even after test failures were managed. Norton et al concluded that cfDNA should not be adopted as a primary screening tool at this time, which would deny women access to cfDNA testing that, relative to sequential screening, has higher patient acceptance, lower false-positive rates, and reduced need for invasive testing. Their policy delays the identification of affected pregnancies and thereby access to safer methods of pregnancy termination.