Molly Sims gained 85lbs during her first pregnancy.
The 43-year-old actress - who is expecting her third child with husband Scott Stuber - admitted she struggled while carrying son Brooks, four, as she had an undiagnosed thyroid condition and saw her weight balloon.
Speaking during People's Mom Talk video series to a group of famous parents including Haylie Duff, Garcelle Beauvais and Soleil Moon Frye, she said: "No one tells you what's going to happen.
"With my first pregnancy, I ended up gaining 85 lbs. and had a bad thyroid problem that no one diagnosed throughout the whole pregnancy until four months later."
And Molly - who also has daughter Scarlett, 17 months, with her spouse - admitted she had false hopes that her weight would just drop again after she gave birth.
She said: "Sometimes, deep down, when you're eating that Sweet Lady Jane on the 405, you think, 'It'll come off when I have the baby.' "
Molly recently admitted she has found her third pregnancy "stressful".
She said: "Being 43 ... we wanted to tell everyone, but we were super nervous about, you know, the testing ... so it's just been really, really stressful, but everything so far is on track."
And the 'Fired Up!' star has been experiencing "crazy" morning sickness and constantly feels nauseous, even though she is now in her second trimester, a period where the symptoms typically start to ease.
She added: "I've been crazy sick.
"I wake up in the morning, I feel like I wanna vomit. I have lunch and wanna vomit, and wanna vomit at dinner ... and then when I go to bed, I still want to vomit.
"I have vomited at grocery stores, I have vomited in my house, I have gagged on my toothpaste."