More mothers with young children are in paid work than in the past. There is a long-running debate on possible negative effects of maternal employment on child development. For the first time, this paper presents an initial comparative analysis of longitudinal data on maternal employment patterns after birth on child cognitive and behavioural development. The paper examines data of five OECD countries with different types and intensity of support provided to families to reconcile work and family life. The evidence suggests that a return to paid work by mothers within six months after childbirth may have negative effects on child outcomes, particularly on cognitive development, but the effects are small and not universally observed. Other factors such as family income, parental education and quality of interaction with children have greater influences on child development than early maternal employment per se.