Background: Early prenatal ultrasound is an important part of prenatal screening in Israel. No studies have described the rate of trisomy 21 [T21] identification at 14–17 weeks gestation.

Objectives: To describe the rate of T21 identification by transvaginal sonograms (TVS) at 14–17 weeks gestation. 

Methods: We conducted a historical prospective study. Since 1986, early TVS of 72,000 fetuses at 14–17 weeks gestation have been prospectively recorded together with prenatal screening data at a private ultrasound center (AL-KOL, Haifa). We calculated the fraction of T21 cases by dividing the total number of cases with abnormal sonographic findings by the total number of diagnosed T21 cases. We also examined the percentage of verified T21 cases that had completely normal prenatal screening tests prior to the early prenatal TVS, thus revealing the contribution of this examination to the existing prenatal screening. Fisher’s exact test was used to calculate odds ratios for each sonographic marker. 

Results: Of 137 T21 fetuses, 123 had sonographic markers on early TVS, yielding a prediction capability of at least 89.87%. Of all T21 cases, 14% had completely normal nuchal translucency/first-trimester screening prior to the abnormal 14–17 week TVS findings. Isolated abnormal sonographic findings, which were found to increase the risk for T21, were common atrioventricular septal canal (odds ratio 88.88), duodenal atresia (OR 88.23), nuchal edema (OR 39.14), and hydrocephalus (OR 15.78). Fetal hydronephrosis/pyelectasis was non-significant when isolated (OR 1), and cardiac echogenic focus was associated with a decreased risk (OR 0.13).

Conclusions: Early prenatal TVS at 14–17 weeks may identify almost 90% of T21 and adds 14% to the identification rate at the first-trimester screening.