This study examines how a mother being the main or an equal earner impacts the relationship stability of heterosexual couple parents, using the UK’s Millennium Cohort Survey. Various theories alternatively predict that such couples experience a higher or lower risk of divorce than male-breadwinner couples. Alternatively the characteristics of these couples may predispose them to relatively higher or lower relationship stability than male-breadwinner couples. Using piecewise constant exponential event history models, we test these propositions between key stages in a child’s life: baby, toddler, school entry and age seven. In some periods, a mother being the main or an equal earner is associated with a lower risk of relationship breakdown than for male-breadwinner couples, and more so within cohabiting than married couples. However, there is a strong tendency for couples to shift over time from mothers being main or equal earners to a male-breadwinner arrangement.