Acknowledgements: the authors would like to thank V.V. Usov, Dr Sci. (Med), Professor at the Medical School of the Far Eastern Federal University, for academic supervision and mentorship in research work.

Author contribution: Mitryashov KV — developing the study concept and design, data acquisition, analysis of the results, manuscript writing, editing; Mitryashov IV — data acquisition, statistical data processing, analysis of the results, manuscript writing.

Compliance with the ethical standards: the study was conducted in accordance with the Order of the Ministry of Health of the Russian Federation dated 28.04.2007 No. 307 “On the Standard of Dispensary (Preventive) Observation of a Child during His/Her First Year of Life”. Parents submitted the informed consent to ultrasound. No approval by the Ethics Committee was required.