The 38-year-old saw her family swell from two to four last May with the birth of Phoenix and adoption of Lukensia.
Now, as she prepares to return to The Biggest Loser this autumn, Jillian has revealed how much motherhood has changed her.
Jillian told Redbook she now understood that being a mother meant had to make sacrifices.
'You can’t have it all, and you have to make some tough decisions,' she said.
'I used to think you could, but you can’t. I didn’t understand that something has to give.
'Now friends will go to Vegas for the weekend and we’re like, “Take pictures.” When Lu looks at me and goes, “Mama, I love you!” I go, “Fft. Who needs Vegas?”'
Jillian's fiancé Heidi Rhoades gave birth to Phoneix last May, the same month they welcomed three-year-old Lukensia to the family.
The timing was a coincidence, following a lengthy adoption process.
Proud parents: Jillian and partner Heidi Rhoades take Lukensia and Phoenix on a walk in Los Angeles
But she explained: 'When we were having kids, I thought, How am I supposed to hide a newborn?
'So we thought, Let's talk about it. It wasn't so much about coming out. It was more about, okay, this is my family.'
Jillian also revealed the couple's thoughts on how they act as parents, saying they try not to be 'helicopter parents'.
'Kids are accident-prone; they're going to tumble,' she told the magazine. 'As long as I know she's not going to die or get badly hurt, it's okay.
'We'd rather her be adventurous, and fall and get back up, than helicopter around her and make her feel fragile.'
Cover girl: Jillian features in this month's Redbook
'We talk about role models for her, like Beyoncé, who has a more curvaceous figure,' she said.
'My taste has changed. I prefer that look. For now, I want to look a little softer.'
Surprisingly, considering her career as a personal trainer, Jillian revealed that she had gained weight since expanding her family.
'I definitely did,' she laughed. 'I went to interview Suze Orman for The Insider, and her wife said, “You’re chubby!.” And Suze said, "You look like a mommy now.”
'I look at my body and I go: “five pounds heavier.” But part of me really likes it.'
But she has been keeping in shape, working out at home with the help of her own DVDs. One thing puts her off, however, the sound of her own voice.
'I haaate it. It’s relentless! I never shut up! And it never stops!
'I’m working you till you’re out of breath, I give you no break, and I’m screaming at you! I found myself screaming back at the TV, “Shut up!” I had to mute it.'
Jillian will rejoin The Biggest Loser this year after leaving in May 2011 in order to bond with her children.