The dynamics of women's labour supply are examined at a crucial stage of their lifecycle. This paper uses the longitudinal employment history records for the 3,898 33-year-old mothers in the Fifth Sweep of the 1958 National Child Development Study cohort in the United Kingdom. Models of binary recurrent events are estimated, which correct for unobserved heterogeneity, using SABRE software. These focus on women's first transition to employment after the first childbirth, and on the monthly transitions from first childbirth until censoring at the interview. Evidence of a polarization is found between highly educated, high-wage mothers and lower-educated, low-wage mothers.