The study included 554 mothers, many of whom worked and had children who attended day-care facilities.
Both mothers and children were fitted with movement and heart-rate monitors that recorded their activity levels for up to a week.
Just 53 per cent of the mothers taking part in the study engaged in 30 minutes of moderate-to-vigorous activity at least once a week, the results published in the journal Pediatrics showed.
Government health guidelines recommend 150 minutes of "moderate intensity physical activity" such as brisk walking over the course of a week.
New parents are known to be less active than their childless peers. Once women become mothers, their activity levels frequently fail to return to pre-parenthood levels.
"There are many competing priorities for new parents and making time to be active may not always be top of the list," Ms Hesketh said.
"If activity in mothers and children can be encouraged or incorporated into daily activities, so that more time is spent moving, activity levels are likely to increase in both."
- PAA