From the University of Bristol's News and Events:

Much controversy surrounds the effects that mothers who go back to work early have on their children's cognitive development. Research by Paul Gregg and Liz Washbrook provides new insights on the links between maternal employment and child outcomes through the experiences of 12,000 children born in 1991 and 1991, the ALSPAC (Avon Longitudinal Study of Parents and Children) cohort.

The research shows that for the majority of children, maternal employment in the first three years of life is not associated with any adverse effects on later cognitive outcomes. Full-time work begun prior to 18 months does have harmful consequences if the family relies solely on unpaid childcare by a friend or relative; the use of formal childcare, however, protects children from these negative effects. And parenting tasks are more equally divided in households where mothers return to work early and children appear to benefit from this greater degree of father involvement.