Materials and Methods: The retrospective cohort study included 25798 pregnant women who underwent NIPS in Russia from January 2013 to December 2018. 21 042 and 4756 participants underwent tests from the Natera laboratory, USA (non-invasive prenatal test 1, NIPT-1) and the Genomed laboratory, Russia (non-invasive prenatal test 2, NIPT-2), respectively.

Results: A high risk of trisomy 21 (T21) was detected in 544 cases (2.59%). The mean age in women at high risk of T21 was 37.3 years. According to the NIPT results, in patients who revealed a low risk of T21, the mean age was 33.8 years (P < 0.001). In 535 cases, invasive prenatal diagnosis was performed. In 7 (1.3%) cases, the presence of T21 in the fetus was not confirmed. In 528 (97.3%) cases, T21 was confirmed by fetal karyotyping. Among women who revealed a low risk of T21 (N = 25 086), in 4 cases (0.015%), fetal trisomy 21 was missed. Thus, the indicators of the effectiveness of NIPS in Russia in relation to T21 are as follows: sensitivity - 99.25%, specificity - 99.96%, PPV - 98.7%, NPV - 99.98%. In the first trimester, in the presence of fetal T21, the level of the fetal fraction of free-DNA is significantly lower.

Conclusions: NIPS has good prospects for implementation in pregnancy management programs and increasing the effectiveness of prenatal detection of T21. The level of the fetal fraction is associated with the presence of fetal T21.