Is Cisterna Magna Width a Useful First-Trimester Marker of Aneuploidy?

To assess whether the cisterna magna (CM) width measured in first-trimester fetuses is a useful marker for aneuploidy detection.

This was a prospective study in 2 different cohorts in a tertiary referral center. The first cohort comprised 913 fetuses from the general pregnancy population during the period 2012-2016 and was used to construct the CM reference ranges applying the λ-μ-σ (LMS) method. The second cohort included 714 high-risk fetuses undergoing chorionic villus sampling during the period 2012-2016. Mean detection rates using the 95th percentile for CM width observed in chromosomal anomaly groups were compared with those obtained in chromosomally normal fetuses.

The 50th percentile for CM ranged from 1.66 to 2.75 mm when crown-rump length (CRL) increased from 45 to 84 mm. Among high-risk fetuses, the following chromosomal anomalies were diagnosed in 125 (17%) fetuses: trisomy 21 (n = 63), trisomy 18 or 13 (n = 21), monosomy X (n = 9), submicroscopic anomalies (n = 11), and other anomalies (n = 22). The mean CM width for euploid fetuses was 2.4 mm (1.13 multiples of the median, MoM). While CM width was significantly increased in trisomy 21 (mean 2.7 mm; 1.23 MoM; p > 0.05), no differences were found in the other anomaly groups. Among the 63 fetuses with trisomy 21, a CM width above the 99th percentile was observed in 23 fetuses (37%).

The new reference range for CM width at 11-13 weeks of gestation did not differ from previous studies. In first-trimester fetuses with trisomy 21, CM width appears to be increased, although its value as an ultrasound marker is limited, because of its detection rate of 37%.