Children whose mothers return to full-time work before they are 18 months old are slower at developing basic learning skills, a new study says.
Striking a balance between work and family life can be difficult
The report on the UK's working mothers said 14% go back to full-time work within the first year and a half of their child's life.
It found that the reading, writing and speaking of these children, between the ages of two and eight, were "negatively affected".
The Bristol University study also found "significant ill-effects" on the development of children who were cared for by a friend, relative or neighbour while their mother worked.
The researchers assessed the development of more than 12,000 children born in 1991 and 1992 at four stages and compared the findings with their mother's working patterns.
The tests took place as part of the ongoing Children of the 90s project, which is monitoring the progress of 14,000 children in the Bristol area since 1991.
The report read: "There are negative effects only for the relatively unusual group of children whose mothers return to full-time work before they are 18 months old.
"The magnitude of these effects is small, and only 14% of mothers do go back to full-time work this early.
"It is only those children whose non-parental care consists solely of unpaid care by a friend, relative or neighbour - such as a grandparent - who experience significant detrimental effects.
"Short periods of care by relatives appear not to be damaging: it is sole reliance on relatives to cover fulltime working that appears to be less
The researchers also found maternal employment in the first three years of life appear to have no adverse effect on later cognitive outcomes.
They said that policies which encourage flexible and part-time working practices, or enable mothers to remain at home for longer after a birth could minimise the negative effects, as could policies affecting paternity leave.
Report authors Paul Gregg and Liz Washbrook said: "Relatively few mothers in our study made use of paid care before their children reached the age of two, probably due to the prohibitive costs.
"The recent increases in financial support for childcare may lead to a shift towards paid care by working mothers."