Women now make up half the British workforce, leading to more pre- school children being looked after in professional care. Many working women, especially those with babies, worry that leaving their small children to the care of others will affect bonds with their child. It seems their concerns may well be valid.
In one of the largest and most controlled studies of its kind, involving 1,300 children and their families, researchers found that longer hours in professional child care up to the age of three correspond with less sensitive interactions between mothers and children.
The findings, published in the Journal of Developmental Psychology, indicate that the more hours a child spends with a childminder the less familiar they are with their mother, and the less "in tune" mother and child are with each other.
Whether the child was looked after in a nursery, at a childminder's house or by a relative, the effect on the relationship with their mother is the same. But the quality of care does have a positive effect on the mother-child relationship.
"This study showed infants and toddlers in better-quality care tended to have slightly better relationships with their mothers than those in lower-quality care," said Dr Martha Cox, senior scientist at the Frank Porter Graham Child Development Center, University of North Carolina at Chapel Hill (UNC-CH), US, and co-author of the study.
The researchers recommended that mothers set aside more "quality time" with their young children to make up for the possible effects of child care.
"Most working mothers spend as much time as they can with their children," said Penelope Leach, childcare guru and author of You and Your Baby.
"The effect of child care on children and families is very complex and is completely dependent on individual family circumstances.
"The people it affects most are the less-privileged mothers who are forced to work long hours and are not able to choose the best-quality child care."
The results were based on data from the National Institute of Child Health and Human Development (NICHD) Study of Early Child Care, a study conducted at 10 different universities. Researchers followed 1,274 mothers and their infants from the time of birth and observed them when the children were six, 15, 24 and 36 months old, both at home and in nursery care.
The children's language and mental development were assessed and the mothers were interviewed about their levels of depression and their toddlers' behaviour.
The variation in the number of hours in child care was related to both the mother's behaviour toward her children and the children's positive engagement of the mother in their interactions. Dr Margaret Burchinal of UNC-CH, who was co-author of the study, said: "We found that on average more advantaged mothers, women with more education and higher incomes, are more likely to be sensitive with their kids and to be working more hours outside the home.
"We only find that working more hours is related to less sensitivity when we compare mothers with similar levels of education and income, but not when we compare mothers regardless of their education and income," she said.
The authors point out that the findings of a small negative association between mother-child interaction and hours of care, and a small positive association with quality of care, may suggest the results of child care are more a product of mothers who use child care than a consequence of the care itself.
The findings may indicate that mothers who are less sensitive to their infants' signals, or who have children who are less engaging, use child care for more hours.
The findings may also suggest that mothers who are more sensitive choose higher quality care.
Dr Margaret Tresch Owen, of the University of Texas at Dallas, and co- author of the study, said: "As a parent, I would take these findings not as bad news, but as encouragement to me to maximise the time I spend with my very young children. More time is better than less time."