Mothers’ longer time out of the labour market due to parental leave has been proposed as one of the main determinants of the gender pay gap. This study focuses on the mechanisms behind the gendered division of care after entering parenthood. By comparing paid parental leave use of biological parents (where mothers gave birth) to adoptive parents (where they did not), we assess to what extend the unequal division of care can be explained by physiological aspects of motherhood or if other explanations, like gender norms or financial motives, can explain these differences. We analyse Swedish register data from 1994 to 2009 on couples whose first child was biological (N = 114,479) or adopted (N = 5,467) (between-family comparisons) and for families who had both adopted and biological children (within-family comparisons; N = 1,033). We find highly similar patterns in the division, length, and timing of parental leave for biological and adoptive children. Both biological and adoptive mothers take the majority of leave (78–82 per cent), the longest leave, and the first leave period. We conclude that persistent norms of mothers as caregivers/homemakers and fathers as breadwinners shape parents’ use of parental leave to a greater extent than factors related to biological motherhood or financial motives.