Women in Sweden are absent from work due to illness to a larger extent than men, and this has been the case over the last 30 years. This project analyses the relationship between family formation and the gender difference in sickness absence. Using detailed individual-level data, we test a number of hypotheses about the impact of first-born children on the gender difference in sickness absence within matched parent couples. We try to answer the following questions: Does the birth of the first child have any impact on the gender difference in sickness absence in the short and long run? Does the effect vary across couples with varying gender income gaps? Is the size of the effect different depending on how equally parental leave is shared within couples? To answer these questions, we use individual-based register data from Statistics Sweden and the Swedish Social Insurance Agency.