RecruitingDiagnostic testHair Cortisol as Marker of Chronic Stress in Preterm and Term Fathers - Fathair-studyThe goal of this observational study is to compare the paternal hair cortisol as a marker for chronic stress in prematurely born children to maturely born children. The main questions it aims to answer are: * How differ the cortisol level between groups? * How change the cortisol level over time? * Are there secondary outcomes associated to the cortisol level of fathers? Participants will give a hair sample to analyse the cortisol level and fill out questionnaires at three time points. At six months of the infant's age, the investigators will also measure the paternal sensitivity.