It's called the "9 months on, 9 months off" rule - nine months to put the pregnancy weight on, and nine months to get back to where you were before.
But of course, even this gentle weight-loss rule isn't always easy to follow.
Especially when you're juggling the demands of a young family - and perhaps work also - on very little sleep.
One woman, however, believes that many mums make excuses for not getting back into shape.
Belinda Norton, 42, is a parent herself and a personal trainer and claims she is in better shape now than she was in her twenties.
Woman explains why she always has sex on first date - and it has one huge benefit
Undoubtedly in amazing shape now, the catalyst came after she got fed-up with her appearance, having gained three stone throughout her pregnancies.
She told Kidspot : "I didn't look healthy anymore.
"I started to eczema again, and I hadn't had it since I was a kid. My energy was zapped."
Belinda, from Brisbane, adds that she "made excuses" for not getting back into shape and had "succumbed" to her body and motherhood.
Woman's warning after she overhears man asking teen for 'dirty photos' on plane
It was her then-husband which sparked her desire to get into shape.
Having shown him a photo of a toned woman and told him she too could look like that, he just laughed.
Now the mum-of-two weighs just under 13 stone, has 54,000 Instagram followers and blogs as Fit Mama.
She urges others: "Enough of the self pity. Enough of the excuse list. Enough waiting.
"Because it is truly OK to change as we are all meant to grow and develop ourself.
Mum's clever Nurofen trick for splinters, stings and glass is a must for parents
"For me the biggest change was NOT my belly roll, it's NOT my new found confidence.
"It's my energy and overall health. I used to poison my body daily.
"I did not even know I was doing it. I just ate because I was tired.
"It was the leftovers from the kids. Plus I didn't get a chance to eat properly, I was too busy with two kids and an endless list. Right??
Woman gives herself food poisoning after making common slow cooker mistake
Belinda also debunks some of the reasons why mums don't exercise as much as they would like to.
"We're not as time poor as what we think.
"It's a myth that exercise will make you more tired. The more we do it, the better we feel.
"We don't need hours to go to the gym. Just get off the phone for half an hour and it's amazing what you can achieve."
