The majority of employed mothers rely on informal care provided by grandparents and their partner.
The findings are revealed in a new book, Children of the 21st century: From birth to nine months. It is based on the first results of the Millennium Cohort study, which is following the lives of around 19,000 babies born in 2000 and 2001 in the UK and Northern Ireland.
Sixty-three per cent of working mothers were not using formal childcare, and of the 37 per cent using formal childcare, some used informal childcare as well.
Nurseries were the most popular choice of formal childcare for working mothers. Twenty per cent of employed mothers used nurseries when their baby was nine months old, 14 per cent used childminders and only 2 per cent a nanny or an au pair.
Working mothers tended to use a mixture of formal and informal childcare, with 22 per cent of those using formal childcare also helped with childcare by a grandparent.
Professor Heather Joshi from the Centre for Longitudinal Studies at the Institute of Education, who is co-editor of the book, said, 'My impression is that most of the mothers returned to work when their babies were between four and seven months.'
When their babies were nine months old, half of the mothers surveyed were working and three-quarters of those with jobs were working part-time. Just 13 per cent of all the mothers surveyed worked full-time.
Professor Joshi said, 'Black African and Caribbean mothers were more likely to be in jobs when their babies were nine months old and were more likely to be in full-time employment.'
The majority of working white mothers were in part-time jobs.
As for fathers, 78 per cent had taken some leave from work when their baby was born, but it was not necessarily paternity or paternal leave, the report also found.
Fifty-three per cent of fathers said they fed their baby at least once a day and 57 per cent said they changed their baby's nappy at least once a day. Sixty per cent said they 'looked after the baby on their own at least a few times a week'.