Funding: the study was supported by a grant from the Russian Science Foundation (project #20-65-46014).

Author contribution: Chagovets VV — study planning, sample preparation, discussion of NMR data processing, manuscript authoring and editing; Vasil'ev VG — sample preparation, NMR analysis; Iurova MV, Khabas GN — collection and characterization of clinical samples, discussion of the results; Pavlovich SV — study research, discussion of the results; Starodubtseva NL — study planning, clinical data processing; Mayboroda OA — study planning and management, manuscript authoring, NMR data processing.

Compliance with ethical standards: the study was approved by the Ethics Committee of the V.I. Kulakov National Medical Research Center for Obstetrics, Gynecology and Perinatology (Minutes #10 of December 05, 2019), conducted in accordance with federal laws of the Russian Federation (#152, 323 etc.) and the Declaration of Helsinki of 1964 with all subsequent extensions and amendments regulating scientific research involving biomaterials obtained from human beings.