Risk factors of periventricular leukomalacia in singleton infants born from 23 rd to 26th weeks of gestation – Retrospective study - Szpecht, Dawid; Wiak, Katarzyna; Braszak, Anna; Surzyn, Emily; Szymankiewicz, Marta; Gadzinowski, Janusz (Pediatria Polska - Polish Journal of Paediatrics, 2017)