Even in couples most likely to believe in sharing parenting responsibilities, mothers still bear significantly more of the child care load, a new study reveals.

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.

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

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

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.

"Both may think they should divide child care responsibilities equally, but mothers still feel a special pressure to show they are being the best parent they can be," said Sarah Schoppe-Sullivan, co-author of the study and associate professor of human sciences at Ohio State.

Kotila and Schoppe-Sullivan conducted the study with Claire Kamp Dush, assistant professor of human sciences at Ohio State. The findings were published online this week in the December 2013 issue of the journal Family Relations.

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.

Most studies of how parents divide child care responsibilities focus on parents of older children, so it was important to see how parents act with infants, Kotila said.

"Our findings show that the differences in how much time mothers and fathers devote to parenting start appearing soon after birth," she said.

Schoppe-Sullivan noted that the habits and routines parents showed here with their first children are likely to continue if they have additional children.

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 both mothers and fathers were highly involved with their children, Schoppe-Sullivan said.

For example, on the non-workdays when their child was 9 months old, parents reported more than 2.75 hours of positive engagement with their infants.

But 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.

"Mothers spent more time on the challenging but vital activities like feeding and bathing," Kotila said.

"The fathers were most involved in positive engagement and accessibility which, while important, may not be as demanding as the routine care. They took on more of a helping role rather than that of the primary caregiver."

But while dads may have room to do more routine care, Schoppe-Sullivan said the results also serve as a warning to mothers about giving themselves a break.

"We have always talked about fathers doing more, but it may be that mothers should do less. They need to relinquish some control," Schoppe-Sullivan said.

Evidence from other research suggests mothers' direct time with their children has actually increased since the 1950s.

"Today's dads are likely doing much more child care than fathers of previous generations. But the are also doing more," she said.

The New Parents Project was funded by the National Science Foundation, with additional support from the Eunice Kennedy Shriver National Institute of Child Health and Human Development.