But the researchers also found that where mothers who return to full-time work at this stage rely solely on unpaid care, children's development suffers 'significant detrimental effects'.
The report, published by the University of Bristol, compares children's educational attainment up to the age of eight against maternal employment.
It emphasises the importance of access to inexpensive and high quality childcare, particularly for very young children.
'The use of paid childcare seems to protect children from any negative effects, and attendance at a nursery may actually lead to better cognitive outcomes than for children at home with a non-working mother,' it says.
The researchers found that only 14 per cent of mothers returned to work full-time before their child was 18 months old. Forty per cent of this group, or 5 per cent of all mothers in the study, relied solely on unpaid care by a friend, relative or neighbour, and it was only in this small group that maternal employment was seen to have any detrimental effect on children's educational attainment.
Co-author Paul Gregg said the research did not demonstrate that paid care was necessarily better than unpaid, but indicated that children's performance at school suffered if full-time working mothers had relied totally on unpaid care by a friend, relative or neighbour. However, there seemed to be no detrimental effects if children were looked after by unpaid carers for short lengths of time. The mothers in the study employed a range of forms of paid childcare, including nannies, childminders and day nurseries.
The researchers found that fathers took on a more active role when mothers returned to work early and that children appeared to benefit from this. The financial gains provided by working mothers also had a positive effect on child development.
Lisa Harker, chair of the Daycare Trust, said the study confirmed that the high costs of care for under-twos 'leaves parents with few options other than to make do with care that appears to be less beneficial', and highlighted the urgent need for the Government to invest heavily in 'formal, high quality, affordable childcare services for very young children'.
The study uses the ALSPAC data (Avon Longitudinal Study of Parents and Children) of 12,000 children born in the Avon area in 1991 and 1992.
The full report, The Effects of Early Maternal Employment on Child Development in the UK, Preliminary results under project: understanding the impact of poverty on children of the 90s, by Paul Gregg and Elizabeth Washbrook, can be downloaded from www.bris.ac.uk/cmpo.