It's a win-win habit for parents to be physically active, note the researchers.
"Encouraging physical activity in parents may also influence their children to become more active, with the added advantage that physically active parents are healthier," they write.
Data came from more than 5,400 kids and their parents who took part in a long-term health study.
The parents completed questionnaires that began during pregnancy.
When children were 11 or 12 years old, they wore a device called an actigraph for a week to record their activity.
The preteens were more physically active if their moms walked briskly or swam while pregnant, and if at least one parent was physically active when the child was 21 months old.
The link between parental and preteen physical activity was "modest," write the University of Bristol research associate Calum Mattocks, MSc, and colleagues.
But findings held when they considered other factors, such as the mothers' age, years of education, and social class.
The study appears online in BMJ, formerly called the British Medical Journal.