The study of 182 dual-earner couples found that the mothers' additional parenting burden started early, when their first child was less than a year old.

The research showed when their children were 9 months old, mothers spent nearly 70 per cent of their time on an average workday (when they weren't working or sleeping) on some type of child care, compared to less than 50 per cent of time for fathers.

"Although the mothers and fathers had similar work constraints, the mothers still invested significantly more time into parenting," Letitia Kotila, lead author of the study and a doctoral student in human sciences at The Ohio State University, said.

The results were somewhat surprising to the researchers because the couples in this study were middle-class and dual earners - just the kind of people that previous research suggested would be most open to equal sharing of parenting duties.

Data for the study came from the New Parents Project, a study at Ohio State that followed couples from the third trimester of pregnancy through the first nine months of parenthood.

In this study, the parents completed time diaries of a workday and a non-workday in which they reported everything they did in a 24-hour period. The parents completed the diaries when their child was 3 months old and 9 months old.

The researchers separated parenting duties into four areas: positive engagement (playing, reading or talking to the child); responsibility (indirect care such as scheduling doctors' appointments); accessibility (watching over the child, but no other parenting activities) and routine care (bathing, feeding, diapering).

The results showed that mothers spent more than twice as much of their parenting time on routine child care than did fathers, even after breastfeeding and pumping were accounted for.