A good early childcare arrangement represents a key element for fertility transitions. It improves the perception of successful parenthood and eases work-family reconciliation. In selecting the most suitable childcare arrangement, individuals will prioritize childcare quality, affordability and availability. Nonetheless, individuals do not freely choose childcare arrangements because of both their preferences and constraints. This article focuses on grandparental childcare provision. By using an instrumental variable approach, it explores the effect of grandparental childcare during the first year of the first born on the risk of a second birth transition among UK working couples. Fertility trends in the United Kingdom have remained fairly stable over the last three decades. Nonetheless, these trends are highly heterogeneous across different subgroups of the population. Fertility postponement seems to have unequal consequences on ‘desired fertility’ achievement among different types of families. Childcare services, especially those for preschool children, are fairly expensive. Along with formal care, grandparental help represents an increasingly popular alternative for childcare provision among Britons. In order to disentangle the effect of grandparental childcare, addressing endogeneity, I use whether both grandmothers are alive as instruments for grandparental childcare provision. The analysis is carried out using the first five waves of the Millennium Cohort Study (MCS). Results show a positive and significant effect of grandparental childcare on the risk of second birth. This effect is slightly weakened by the level of income.