This report presents the findings of research into how and when differences in work behaviour between men and women develop, focusing on the evolution of the gender gaps immediately after childbirth and during the initial years of family development. The analysis presented focuses on two crucial periods in family development: when a new baby arrives and when a child starts school. The study uses two data sources: the first thirteen waves from the British Household Panel Survey covering the years 1991 to 2003 and the first five waves from the Families and Children Study covering the years 1999 to 2003.