Funding: the study was funded by the Russian Foundation for Basic Research, grant reg. number 2004-60093.

Author contribution: Lomova NA — clinical data management, manuscript writing; Chagovets VV —mass spectrometry assay, statistical analysis of the data, manuscript editing; Dolgopolova EL — collection and processing of clinical samples in “red zone”, statistical analysis of the data; Novoselova AV — mass spectrometry assay, processing of spectral data; Petrova UL — collection and processing of clinical samples in “red zone”; Shmakov RG — clinical sample collection and data management, manuscript editing; Frankevich VE — concept, data management, writing and editing of the manuscript.

Compliance with ethical standards: the study was approved by Ethical Review Board at the V.I. Kulakov National Medical Research Center for Obstetrics, Gynecology and Perinatology (protocol № 13 of 10 December 2020) and carried out in compliance with Declaration of Helsinki, the International Conference on Harmonization (ICH) Good Clinical Practice (GCP) Guideline, and Federal Law № 323-FZ “On Basics of Health Protection of Citizens in the Russian Federation” of 21 November 2011. Informed consent for the study was provided by all participants.