Kourtney Kardashian gained 40lbs with each of her pregnancies.

The 'Keeping Up With the Kardashians' star - who has kids Mason, 10, Penelope, seven, and Reign, five, with former partner Scott Disick - has reflected on her experiences of losing her baby weight, admitting it was different each time.

Writing for her lifestyle website Poosh, she recalled: ''It wasn't until Reign was probably four months old that I felt this desire to be in the best shape of my life.

''I gained exactly 40 pounds with all three of my pregnancies, but my body and my experience after having each of my kids was so different.

''I was in a different place mentally, emotionally, and physically, even if by just a couple years.''

The 41-year-old beauty warned women to be wary of ''unrealistic'' expectations about getting back in shape after birth.

She said: ''The most important thing is to listen to your body and to do what you're doing for you, not for society's standards of getting your body back, because they are unrealistic.''

When Kourtney had Mason, she turned to walking and running to shed the pounds, before opting for ''mellow workouts and a lot of yoga'' after Penelope was born, and then eventually discovering High-Intensity Interval Training (HIIT) workouts.

She said: ''It's a lot of short, super intense bursts, like jump lunges, squats, or running up stairs, and it burns crazy calories and builds strength.''

Kourtney credited her ''super motivating'' trainer Don for helping her love the sessions and she admitted she ''couldn't wait'' to get back to them after Reign was born, especially because of the boost they gave her mental health.

She said: ''I couldn't wait to get back on my high-intensity schedule after the pregnancy, and it felt so good to jump back into it.

''Especially because I was going through the breakup with Scott, I found that these workouts helped crush my anxiety.

''HIIT workouts are really what made the biggest difference. It's hard at first, but I noticed once I became used to that sort of intensity, I craved it.''

The brunette beauty also advised new mothers to continue ''breastfeeding for as long as you can'' because of the positive lifestyle habits it encourages.

She explained: ''It forces you to eat super clean for your baby, drink much less alcohol and caffeine, and hydrate with a ton of water.''