Kim Kardashian, who announced she was pregnant with baby number two earlier this month, openly struggled with her weight during her first pregnancy with daughter North West. Making sure she stays on track this time around, the 34-year-old has hired a nutritionist and is also again working out with celebrity trainer Harley Pasternak as part of her regimen to keep her looking and feeling healthy.

"Kim was very active throughout the first pregnancy, and I think she'll be just as active through this one," Harley told HELLO! at the smartwater up up up lounge in New York. "With your first one, you deal with nausea and all sorts of stuff. I think she's been pretty good through this one. She knows what to expect. A lot of women, when they're pregnant for the first time, are scared to do anything, so now it's old habit for her."

Keeping the star moving and safe throughout her second pregnancy, Harley, who has trained stars from Katy Perry to Lady Gaga, gave us his rules for working out with his pregnant A-list clients.

"It's very important to be sure their heart rate doesn't get above 140 beats per minute," he said, adding that staying well-hydrated and exercising in a cool, well-ventilated room is key. "So, no hiking in Runyon Canyon or working outside in the backyard in Calabasas."

When it comes to exercise, pregnant women need to be cautious but shouldn't be afraid to stay active. "The only point a woman should stop working out during their pregnancy is if their doctor tells them," concluded Harley. "I think Megan Fox worked out the day before she gave birth."

There are guidelines, though, depending on what stage of pregnancy you are in. When it comes to his clients, "We do everything we would do when they're not pregnant – except when we start the second trimester, we eliminate all exercises where they are lying on their back," he explained. "I encourage [Kim] to walk at least 10,000 steps a day."

Though ab exercises are limited as the baby grows, "Resistance exercise is important. We are doing lunges and hamstring curls for toning."

So if you're a mom-to-be, what can you do to be like Kim? Based on his famous 5-Factor Fitness plan, Harley encourages his pregnant clients to do a few toning exercises several days a week: complete two sets of 10-20 reps of moves like standing side bends, bent-over rows and sumo squats to target the whole body, using 3, 5 or 8-pound dumbbells.