A mother's post-baby weight is due to her lifestyle choices and not to the pregnancy.
The study found that mothers keep the weight on by eating food off their child's plate or sitting down for longer periods of time.
They also suffer cravings as a result of sleep deprivation, which can make them eat junk food even if they want a healthy diet.
If women want to lose the weight they should cut the calories and work out more, reports DailyMail.
The study suggests that celebrity mothers who use expensive fitness coaches to get their figures back weeks after giving birth may be on to something.
Researchers from the University of Michigan looked at data for 30,000 women who had given birth between one and four times.
All the women gained 1.94lbs a year on average due to their age, but there was a sharp difference between those who gave birth and those who did not.
By the time their children were toddler age, mothers who had given birth had put on at least one pound more each year than those who were childless.
Lead author Dr Olga Yakusheva said that the difference was because "mothers tend to put the needs of their children first, so they might not be exercising or taking care of themselves."
She said: "It might also be little things like finishing the food on their child's plate or spending more time sitting with their kids reading or watching a movie.
"Many women really crank up their diet and exercise for a short time to get back to their pre-pregnancy weight, and often get discouraged by the results.
"But it's much better to take a holistic approach focused on a long-term healthy lifestyle before, during and after pregnancy."
Celebrity mothers have long drawn admiration - and criticism - for getting their figures back weeks after having a baby.
Beyoncé appeared on the cover of People magazine four months after giving birth to daughter Blue Ivy in 2012 and said she felt "more beautiful than ever".
Gwyneth Paltrow worked out two hours a day to get back in shape after the birth of her son Moses in 2006.
Victoria Beckham reportedly followed the Five Hands Diet - eating only five handfuls of food a day - to slim down for New York Fashion Week two months after the birth of daughter Harper in 2011.
Dr Yakusheva, an associate professor at the University of Michigan School of Nursing, said that during her first pregnancy she gained 70lbs, and during her second 60lbs, despite cutting calories and weighing herself every day.
She said: "I felt terrible. Many women feel that anxiety about gaining pregnancy weight because they're already anticipating pressure to lose the weight, from others and themselves.
"Understanding the demands of motherhood and age-related weight gain is important for promoting positive expectations of body image after pregnancy. As long as women are healthy, that is what matters."
Dr Yakusheva said that doctors should advise women about age-related weight changes, and make them aware of subtle ways parenthood could exacerbate these changes over many years.
Erin Palinski-Wade, who offers nutrition advice for families on MommyhoodBytes.com, said that many mothers could relate.
She said: "After having a child, it's not so much that you can't lose the weight, but you may have less time to go to the gym, less time to food shop, and less time to prep healthy meals.
"Sleep deprivation itself can increase food cravings and appetite, which certainly doesn't help with weight loss. So you might just be too sleep-deprived to care to exercise or eat well."