They found that mothers with older children (ages 5-18) were physically active for about 11 hours per week less in 2010 than in 1965. Among moms with children younger than 5, the decline was nearly 14 hours a week. Today, the two groups are physically active for 20.9 and 29.7 hours each week, respectively.

Though unemployed mothers might be thought of as having more time for exercise, physical activity has decreased more among mothers who don't work than among those who have full-time jobs. (Though moms who don’t work still spend more total time being physically active than those who do).

Mothers with older children have also increased their sedentary time by 7 hours a week since 1965. The increase was 5.7 hours a week for mothers with younger children. Again, unemployed mothers saw greater increases in sedentary time. As a result, mothers today burn between 1,238 and 5,835 calories less each week than mothers did in the 1960s. A pound of fat is roughly 3,500 calories.

Of course, much of the decline has to do with housework. Dishwashers didn’t become a staple of middle-class homes until the 1970s, and clothes washers and dryers were out of reach for many until the 1980s. Few women today would likely say that their preferred form of exercise is scrubbing shirts on a washboard.

But the other side of the technological-advancement argument is the ubiquity of screen-based entertainment: Physical activity time reached a “nadir” in the 1990s, according to the report authors, just as the Internet became popular and TVs plummeted in price.

If mothers set examples for children, they’re increasingly modeling a screen- and car-based existence, which probably doesn’t bode well for either their own or their kids' health. Not only does a pregnant woman’s body mass index impact the health of her fetus, the report authors say kids raised by sedentary caregivers are less likely to have healthy habits themselves.

“An emerging body of epidemiological and experimental evidence suggests that maternal behaviors may also play a role in determining the developmental trajectories ... that determine the risk of obesity in children,” they point out.

It’s not clear what the solution is here. As Liza Mundy wrote in The Atlantic, among couples with children, when both spouses work full-time, women do more housework, childcare, and other family-related services each week, leaving many feeling like they already come home to a “second shift.” Many unemployed mothers, on the other hand, likely don’t have the resources to sign up for Mommy & Me Yoga, or even to pay for an hour of babysitting so they can jog around the block. There was very little good about the days when women were essentially housebound, unpaid manual laborers, but ironically that lifestyle might have been better for their physical fitness.

*The sample of mothers with children were: 586 for the 1960s, 1050 for the 1970s, 539 for the 1980s, 1313 for the 1990s, 10,103 for 2003-2005, and 13,846 for 2006-2010.