Objective To examine the effects of parental employment in the early years on child socio-emotional behaviour at age 5 in a recent birth cohort study in the UK.

Design Prospective cohort study.

Setting The Millennium Cohort Study (MCS): a large, representative sample of children born in the UK between September 2000 and January 2002 (n=18 819 at sweep 1).

Participants Singleton births in households in which a mother was present in the first three sweeps of the MCS, when participants were 9 months, 3 years and 5 years. Analysis was restricted to white children as there was large ethnic variation in maternal employment, but inadequate power to stratify by ethnicity.

Main outcome measure The Strengths and Difficulties Questionnaire (SDQ) with clinically relevant cut-points for problem behaviours. The SDQ is a widely used instrument for assessing socio-emotional difficulties in children.

Methods Data on parental employment across the three sweeps were used to investigate: (i) whether children whose mothers were in paid work during their first five years were more likely than children whose mothers were at home full-time to display adverse behavioural symptoms at age 5, independent of maternal education, mental health or economic position; (ii) whether effects of maternal employment on child socio-emotional development were cumulative in nature, or whether children were more sensitive to the effects of maternal employment during their first year; and (iii) the effects of different types of parental work arrangements on child socio-emotional behaviour at age 5.

Results No evidence of detrimental effects of maternal employment in the early years on subsequent child socio-emotional behaviour was seen. There were significant gender differences in the effects of parental work arrangements on behavioural outcomes. Girls whose mothers were not in paid work during their first 5 years were 77% (95% CI 1.21 to 2.57) more likely to have behavioural difficulties at age 5 than girls whose mothers were in paid work throughout their early years, independent of maternal characteristics and household income. For boys this was not the case, but boys in two-parent households in which their father was not in paid work for at least one period during their first five years were at an increased risk for behavioural problems at age 5. The most beneficial working arrangement for both girls and boys was that in which both mothers and fathers were present in the household and in paid work, independent of parental educational attainment and household income.