Polish mother and child cohort study - defining the problem, the aim of the study and methodological assumptions - Polańska, Kinga; Hanke, Wojciech; Gromadzińska, Jolanta; Ligocka, Danuta; Gulczyńska, Ewa; Sobala, Wojciech; Wąsowicz, Wojciech (Nofer Institute of Occupational Medicine)